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McKkON & SCHOFIEIiD, Propkietors, 

8 MnRRAT Street. 

Copyright 1896, 


The following pages have been written witli a 
view of offering some assistance to two classes of 
readers, namely, those who desire to add to their 
stock of general information, and others who 
wish to pursue for themselves a line of investiga- 
tion and experiment in the fascinating domain of 
the mysterious X rays. 

Many of those who will desire to make a 
special study of the subject or make practical use 
of Prof. Roentgen's discovery will probably have 
had no occasion to acquire any special knowledge 
of electrical matters, and will, therefore, be at a 
loss to know what apparatus is needed, having 
due regard to the conditions by which they are 

The aim has been, therefore, to make all ex- 
planations as full as possible and as simple as is 
consistent with statements of technical matters. 
A careful endeavor has also been made to include 
in the following chapters suflicient information 


8 Preface. 

to enable tlie layman to choose tlie apparatus 
most suitable to bis needs as well as to operate 
it and obtain successful results tberefrom. 

W. H. M. 
October 15, 1896. 



Although the main object of this book is to 
present to the reader a practical explanation of 
apparatus and metbods employed in producing 
and utilizing tbe X rays, the present chapter 
will be treated as a general introduction of the 
subject. The object in so doing is to set forth a 
general outline of Professor Roentgen's discovery 
and, incidentally, to correct certain popular mis- 
conceptions that have arisen as to some of the 
phenomena and the methods of their utilization. 

Looking back along the years of the present 
century, we see within their bounds the greatest 
development and progress that has blessed the 
world since the beginning of time. A mere enu- 
meration of the discoveries and inventions con- 
tributing to comfort, safety, and convenience that 
have been made since the year 1800 would fill 
many large volumes. If we should single out 
from this host only a few of those that are con- 
spicuous by their greatness, such as the Intro- 


lO The A B C of the X Rays. 

duction of Illuminating Gas, the Steam Engine, 
the Railroad, the Steamboat, the use of Anaes- 
thetics, the Electric Light, (Arc and Incandes- 
cent), the Electric Railroad, the Electric Trans- 
mission of Power, and the X rays, we cannot 
but be powerfully impressed with the wonderful 
progress made within the past years of the nine- 
teenth century. 

It seems fitting that the discovery of the won- 
derful phenomena of the X rays should be made 
as the hands upon the clock of the century are 
completing their round, as it was the logical re- 
sult of research and experiment founded upon 
the most advanced kind of intellectual and scien- 
tific study and attainment. 

How far reaching this discovery may ultimate- 
ly prove to be in the arts and sciences it is im- 
possible at this time to predict, but within the few 
months in which it has become known and prac- 
ticed, the wonderful effects have been practically 
applied to the alleviation of much suffering, and 
we have at this moment at our command a mys- 
terious power for good that it would have seemed 
sheer folly to even hope for only one short year 

The discovery of the X rays was made towards 
the end of the year 1895 by Professor William 

The A B C of the X Rays. 1 1 

Konrad Roentgen, professor of Physics at the 
Royal University of Wurzburg, in Germany. 
He had for many years made a study of phenom- 
ena arising from the action of currents of elec- 
tricity in glass tubes more or less exhausted of 
air, known as Geissler, Crookes, or Vacuum tubes. 
In the" year 1894 some new phenomena in this 
line of experiment had been noted by Professor 
Lenard, an assistant of Hertz. 

In the course of a study of these phenornena 
by Professor Roentgen, he discovered that an 
effect was produced by the rays emanating from 
a Crookes tube, which was being excited by an 
electric current, similar in many respects to the 
effect that would be produced by rays. of light, 
but with the wonderful diflference that the new 
rays would penetrate flesh, blood, and muscle and 
many thicknesses of paper, cloth, leather, wood, 
rubber and other substances which are opaque to 
ordinary light rays. 

The first announcement of this great discovery 
was received by the world with incredulity and 
some amusement, but this was quickly turned to 
astonishment and wonder by subsequent an- 
nouncements of practical results having been 
obtained by other scientists on following t,he 
methods employed by Professor Roentgen. 

12 The A BC of the X Rays. 

It is not deemed necessary to enter into a more 
detailed account of the history of this discovery 
than the outline above given, as the newspapers, 
magazines, and technical journals have already 
given much space to the subject. The following 
pages will therefore be devoted to a brief expla- 
nation of the X ray phenomena, followed by 
a description of the various items of apparatus 
employed and of the methods of using the same 
to produce the effects which are now becoming 
so generally known. < 

The generation of the X rays is at the present 
time wholly electrical. It is possible that there 
may later be discovered some other means of 
producing them, but at this day there seems to 
be no well authenticated fact to prove that there 
has been any other source of generation dis- 

Like the electric current itself, the mysterious 
X rays are invisible ; their presence being known 
only by the effects produced under proper con- 
ditions. There has been a popular misconcep- 
tion that the X rays proceed from a glass vacuum- 
tube giving a light of dazzling brilliancy, but such 
is not the fact, for very frequently the vacuum- 
tube under excitation will glow, or fluoresce, 
so faintly that the whole illumination would not 

The A B C of the X Rays. 13 

be sufficient to enable one to tell the time by a 
watcli and yet there might be rays of such 
strength as to affect a photographic plate in a 
few seconds through a piece of wood, cardboard, 
or rubber of considerable thickness. 

Although the X rays in their effects closely 
approximate ordinary light rays, there are such 
differences that scientists have not as yet been 
able to properly classify them. Their exact 
nature is at present unknown. Professor Roent- 
gen himself gave the new rays the name of X, 
(or unknown), rays, by which name they have 
since been generally designated. Research and 
experiment will, however, possibly result in 
ultimately determining the question of their 
proper classification. The most recent investi- 
gations as to the nature of the rays tend to leave 
little doubt that they are transverse vibrations 
in the ether, like those which constitute light 
and radiant heat but of very much higher pitch 
or excessively short length of waves. 

One instance will suffice to make the above 
differences clear to the reader. - It is well known 
that rays 'of light may be deflected, refracted or 
condensed, but not so with the X rays. They 
proceed only in straight lines normal to the point 
at which they are produced. Take, for instance, 

14 The A B C of the X Rays. 

an ordinary magnifying glass whicli is simply 
a glass lens of convex shape as shown in Fig. i. 
The rays of light proceeding from A strike the 
lens and are condensed- and brought to a focns, 
or point of meeting, on the other side of the lens. . 
This is one of the principles applied in the ordi- 
nary photographic lens, making it possible to 
condense the light rays proceeding from a large 
area and project them in greatly reduced form 
upon a sensitized plate in the camera. If, on the 
other hand, such a lens should be placed in the 
path of the X rays, they would proceed through 
it in straight lines (if indeed they could pene- 
trate the glass at all, glass being more or less 
opaque to the rays as we shall presently learn), 
and would neither converge nor diverge. 

Thus far we have found : 

(i) that the generation of X rays is from 
electrical sources. 

(2) That the X rays are invisible. 

The A B C of the X Rays. 15 

(3) That they proceed in straight lines and, so 
far as present discovery goes, cannot be reflected, 
refracted, or condensed, but can be diffused. 

Let us now take a preliminary glance at one of 
the methods employed in producing the rays, 
reserving more complete details for a further 

The appliances used consist of (i) some form 
of apparatus which will give a current of electri- 
city of very high potential, as for instance, a 
Ruhmkorff induction coil ; (2) a mechanical con- 
trivance called a " Contact breaker " for inter- 
rupting with great- frequency the continuity of 
the circuit ; (3) a condenser for accumulating and 
rapidly discharging accumulations of electric 
energy ; (4) a Crookes vacuum tube ; and (5) a 
battery, or other source of electric current. 
More particular explanation of these various 
items of apparatus will be found on other pages 
of this book. 

If the electric current be applied to the coil 
with contact breaker and condenser attached, and 
the secondary terminals of the coil be separated 
within certain distances, leaving only the air 
between, bluish-colored, snappy, lightning-like 
sparks will jump across from one terminal to the 
other at such frequent intervals as to appear like 

1 6 The A B C of the X Rays. 

a continuous stream. The length of these sparks 
will depend largely upon the amount of wire upon 
the coil. In ordinary coils for this work the 
sparking capacity will vary from one to twelve 
or more inches in length. These sparks are of 
the character just described only when they 
occur in the open air, and their curious zigzag 
form is said to be due to the current taking a 
path through the particles of air having the least 

Suppose, however, that we take these two 
secondary terminal wires from the coil and seal 
them into a glass bulb or cylinder so that the 
ends of such wires will project therein. These 
ends would thus be known as " electrodes." If 
the bulb were not exhausted of air, the same 
character of discharge will take place between 
the two electrodes when the coil is energized by 
an electric currrent. 

Now, if the air be partially exhausted from the 
bulb so as to make a moderately low vacuum, say 
a pressure of three thousand one-millionths of an 
atmosphere (an atmosphere being equal to a 
pressure of 15 pounds to the square inch), a very 
different appearance will be noted when the coil 
is once more energized. Instead of a stream of 
sparks jumping across from one electrode to the 

The A B C of the X Rays. 17 

other, the whole bulb will become filled with a 
purplish-colored cloud having a rapidly vibrating 
or travelling movement. This occurs by reason 
of the molecules of air remaining in the bulb 
being driven off from one electrode towards the 
other, and coming into violent contact with each 
other, the force of the collision of these molecules 
produces light. 

If now the tube be exhausted to a high degree 
of vacuum, say, to one one-millionth of an atmos- 
phere, a new set of phenomena is developed on 
discharging the current from the coil into the 
tube. The purplish cloud has disappeared and 
the interior of the tube is clear, but the glass 
itself has assumed a beautiful fluorescence caused 
by the bombardment against it of the remaining 
molecules of air. This is now a Crookes vacuum 

The fluorescence will be most noticeable and 
brilliant at that part of the glass tube that is op- 
posite the negative, or cathode, electrode, and it 
is from this part of such a tube that the X rays 
proceed in greatest abundance. We shall see in 
a later chapter that modifications of this simple 
form of Crookes vacuum tube have recently been 
made, providing for a special bombardment sur- 
face, within the tube, with the result of obtaining 

i8 The A B C of the X Rays. 

a mucli greater abundance of the rays, together 
with less diffusion and, consequently, a greater 
degree of sharpness of definition, in the shadows 
produced by them. 

We find, then, that the X rays are projected 
from a place or point in a highly exhausted glass 
tube that is directly opposite the negative, or 
cathode, electrode, and that this place or point 
may, but need not necessarily be, the glass of 
the tube itself. 

Having brought the reader to the point where 
the X rays are produced, let us now see in what 
manner and by what methods we can ascertain 
definitely that they are present. 

It requires no special methods to ascertain 
when ordinary light rays (sun or artificial) are 
present, for it involves only a radical difference 
which is apparent to every one who is possessed 
of eyesight, namely, the difference between dark- 
ness and light. The fact of the X rays being 
invisible but producing some effects similar to, 
and in some aspects more powerful than, effects 
produced by light rays, presents apparently a 
somewhat paradoxical state of things. 

Comparisons with known standards may in 
many cases be advantageously made in explain- 
ing new phenomena, but in this case there are 

The A B C of the X Rays. 19 

no standards by wliicli the new set of phenomena 
incident to the X rays may be strictly comparable. 

We shall endeavor, therefore, to make the ex- 
planation in the simplest possible manner. 

There are at present only three ways in which 
the presence of the X rays can be ascertained. 
One of the strongest characteristics of X rays is 
their power of discharging electrified bodies or 
surfaces exposed to them. This, in fact, is the 
most sensitive way of detecting them. Another 
way is by means of the ordinary sensitized 
photographic plate, and the other is through the 
fluorescence of certain crystalline chemical salts 
when brought within the influence of the rays. 

It will be necessary only to discuss the latter 
two, and taking these in their order, we will first 
deal with the photographic plate, and at the 
start it will be well to .correct the popular error 
that an ordinary photographic camera is used in 
obtaining pictures with the X rays. A camera 
is hot necessary, in fact no use could be made of 
it in this connection, as the rays cannot be con- 
densed or refracted. 

A picture of any object obtained by means of 
the X rays is not a photograph, strictly speak- 
ing, although it is made upon a sensitized plate 
and developed and printed in the manner usually 

20 The A B C of the X Rays. 

followed in making photographs. As defined in 
the dictionaries, photography is the " fixing of 
an object or objects by -chemical rays of light 
upon certain salts sensitive to light," and as 
~l ordinarily practiced, includes 
the use of lenses by which the 
objects photographed may be 
focussed on the sensitized plate 
on a smaller or greater scale. 
An X ray picture of any object, 
Photography. (or " radiograph " as it is gener- 
ally termed), cannot in the first instance be made 
any larger or smaller than the object itself, 
because lenses cannot be successfully used to 
condense the rays, and therefore, a radiograph is 
practically a \\ie.-size. shadow picture of the object. 
A radiograph, once taken, can, however, h&photo- 
graphed by rays of light in the ordinary way ; 
and thus a new picture of the object, increased 
or diminished in size, may be obtained. 

If we should take a sensitized photographic 
plate and place an opaque object upon it, as, for 
instance, a hand, and expose it for a few seconds 
in daylight, the rays of light would not reach 
that part of the plate covered by the hand. Upon 
developing the plate there would be seen clear 
the shape, or shadow, of the hand, while all 

The A B C of the X Rays. 2i 

around it the plate would be dark owing to the 
rays of light having affected this portion. No 
part of the bones, muscles, or other interior 
portions of the hand would be visible upon the 
plate, as the rays of light would not penetrate 
the flesh and blood, all portions of the hand be- 
ing to a very great extent opaque to light rays. 
Such a picture as this would be termed 2^. photo- 
graphy having been obtained through the agency 
of light rays. A print from this photograph 
made upon" paper in the regular way would show 
'simply a black shadow of a hand, while all 
around it would be clear white, just as a hand 
might be seen outlined upon a white window 
shade if a light were placed behind it. That 
part of the photographic plate which in prints 
from it is white is the part that has been directly 
acted upon by the rays of light. 

The X rays allowed to fall directly upon a 
sensitized photographic plate will affect it in the 
same way as it would be affected by light rays. 
A plate so exposed would, after development, 
yield prints of a clear white. 

Now, for comparison, let us expose two sen- 
sitized plates, one to ordinary light rays and the 
other to the X rays, first placing over each plate, 
so as to cover it entirely, a piece of wood, a book, 

22 The A B C of the X Rays. 

a sheet of rubber, or a tbin sheet of the metal 
aluminium. On developing and printing these 
two plates we find that the one exposed to the 
light rays will print black, thus showing that 
these rays have not penetrated the substance in- 
terposed. The development and printing of the 
one which was exposed to the X rays will, how- 
ever, result in a clear white print, proving there- 
by that the plate was acted upon and that the 
substances above named vsrere no obstacles to the 
passage of the X rays. 

Although the X rays will penetrate the above 
named articles, and many others, it is noted 
that they do not readily penetrate all substances, 
and that among those that are fairly opaque are 
bone, glass and almost all the metals. By reason 
of that fact we are enabled to make examination of 
the bones in a living body and we may also locate 
foreign opaque substances which may by accident 
have entered the human frame. 

I,et us now expose one more sensitized plate 
by way of illustration ; this time to the X rays. 
We will wrap it up in black paper, or enclose it 
in a plate-holder, sliding a cardboard or rubber 
cover over the latter Now we will place it on a 
table, lay a hand upon the covering of the plate 
and produce the X rays by means of a suspended 

The A B C of the X Rays. 23 


Crookes tube. In a few minutes tlie excitation 
of the tube is stopped and tbe plate taken away 
and developed. Upon printing this we find that 
that part of the plate not covered by the hand is a 
clear white. We also find that the X rays have 
penetrated entirely, the muscles, and to a great- 
extent the flesh, blood, and nails, leaving only a 
faint shadow of the outline of the hand. The 
rays have not, however, penetrated the bones, 
consequently the plate immediately beneath them 
has not been affected (or, if at all, very slightly), 
and therefore, as they show in dark relief, we 
have a strong picture of the bones of the hand. 

24 The A B C of the X Rays. 

Had there been a piece of metal sucJb, for in- 
stance, as a bullet or a needle, embedded in tbe 
hand, it would also have shown, these substances 
baing also opaque to the X rays. 

Thus we can to-day obtain pictures of the bony 
structure of any part of the body and show the 
presence not only of foreign substances of a 
metallic nature, but also of calcareous deposits 
or other substances that are opaque to the X 
rays. Broken bones, or those improperly set 
after being broken, as well as dislocations, can 
also be disclosed as clearly as if the flesh were 
stripped from that part of the body in which the3' 
are located. 

A great many experiments have been tried and 
much thought has already been expended in the 
direction of obtaining clearly defined radiographs 
of the heart and other organs, but as these are 
not sufl&ciently opaque to the X rays it cannot 
be said that entire success has yet been achieved 
in this direction. 

A recent cable from abroad announces that 
a radiograph of the stomach and intestines of a 
person has been successfully obtained by having 
the person first drink a quantity of a harmless 
solution of a chemical nature impervious to the 
X rays and then making the usual exposure. 

Made by the author with Thomson Double-focus Tube and 


Time of Exposure z minutes 50 seconds. 

Distance from Tube 12 inches. 
Inductorium working on 5 inch spark. 

The A B C of the X Rays. 25 

Thus far we have shown only one way of as- 
certaining the presence of X rays, namely, by 
their effect upon a sensitized plate. There is 
another and quicker way, however, which arises 
by reason of the fact that certain chemical salts 
possess the peculiar property of fluorescing when 
brought within the influence of the X rays. 
This fluorescence is not brilliant, but merely 
presents to the vision a moderate glow when the 
eyes are shielded from other light. It is not an 
easy matter to find an apt comparison with 
something familiar to every one, but perhaps this 
fluorescence is more nearly comparable to a 
ground-glass window pane at night, having be- 
hind it, at some little distance away, an artificial 
light of moderate candle power. 

The idea of the X rays, which are themselves 
invisible, having the property of causing this 
fluorescence is somewhat of an abstruse concep- 
tion for the layman, but probably a further simple 
comparison will tend to an easily comprehended 
and reasonable view of this. 

It is well known that a diamond is a crystal 
and that in a perfectly dark room it will not show 
any light nor the beautiful colors for which it is 
esteemed. Exposed to rays of light, however, it 
at once becomes a sparkling gem. The chemical 

26 The A B C of the X Rays. 

salts which exhibit fluorescence in the X rays 
are all crystalline, but so far as their fluorescent 
property is concerned, they may be said to be 
always in the dark until they are brought into 
the presence of the X rays, when they at once 
light up, as the diamond does in light rays, 
although not with the same brilliancy or color 

It must be remembered that this comparison 
is not put forth as an explanation of the phenom- 
ena in question, but, as stated above, merely to 
assist the reader in forming some sort of an idea 
which may lead to a fair appreciation of fluo- 
rescence as observed in the Fluoroscope. 

This peculiar property of these crystalline 
chemical salts has been availed of for the purpose 
of practical and instantaneous observations by 
means of a device called the Fluoroscope,. which 
will be described in detail in a later chapter. 

This, fluorescent property was known three 
years ago and a screen covered with the cry- 
stals was used by Lenard in his investigations. 
Roentgen also matje use of a fluorescent screen 
about a year ago. Similar screens were also used 
by Professor Blihu Thomson in the United States 
very early in the present year, very soon after 
the announcement of Roentgen's discovery. The 

The A B C of the X. Rays. 27 

knowledge of the fluorescent screen became gen- 
eral some months ago, and subsequently Mr. 
Bdison put it in the convenient form, now known 
as the Fluoroscope. 

It will be sufiicient to note at this point that 
opaque objects placed between the Crookes tube 
and the fluorescent crystals will prevent the X 
rays from striking that part of the crystals 
covered by such objects and, consequently, the 
crystals will not exhibit fluorescence at that spot. 
For instance, if a fluorescent screen be brought 
within the influence of the X rays and a hand 
be placed between the source of rays and the 
screen, the X rays pass right through the flesh, 
blood, veins, and muscles but do not penetrate 
the bones. Inasmuch as the crystals reached 
by the X rays become fluorescent, the screen 
glows except in the places not affected by the 
rays, and we therefore see in dark shadowy out- 
lines the shape of the bones in the hand as they 
are at that present moment. When the genera- 
tion of the X rays is stopped, or the fluorescent 
screen removed from the sphere of their influ- 
ence, the fluoresence of the crystals ceases, not to 
appear again until once more exposed to the 

We believe that the reader will now have 

^8 The A B C of the X Rays. 

gathered a general idea of the phenomena in- 
volved in Professor Roentgen's wonderful dis- 
covery, and we shall now proceed to describe 
more in detail the various items of apparatus 
used to produce the effects above noted and 
endeavor to give some hints as to its manipula- 

The A B C of the X Rays. 29 



The electric current necessary to produce tlie 
proper excitation in a Crookes vacuum tube 
should be possessed of two important qualifica- 
tions. First, it should be of very high potential, 
and, second, it should preferably be an alternat- 
ing, interrupted or intermittent current. 

If we were to attach a Crookes tube direct to 
two of the conductors forming part of an electric 
lighting circuit no effect would be perceptible. 
We might take away these conductors and con- 
nect to them ten or twenty incandescent lamps 
of from 16 to 100 candle power each, and the 
current flowing in the wires would light up all 
these lamps to full candle power, although it would 
not excite one Crookes tube which would require 
under proper conditions less than one five-hun- 
dredth of the actual electrical energy needed to 
illuminate the lamps. 

There are two reasons for this. First, the 

30 The A B C of the X Rays. 

current (let us say, continuous, or direct, at no 
volts) did not fulfil the qualifications referred to 
in the first paragraph ; and second, that in the 
incandescent lamps the current has continuous 
paths of comparatively low resistance in which 
to travel, while in the Crookes tube the path in 
which the current may pass is exceedingly 
meagre, consisting merely of the molecules of 
air that are left in the tube after exhausting 
it to a degree where it contains only about one- 
millionth of an atmosphere. In this latter case 
it might be imagined that the current on enter- 
ing the tube by one conductor is obliged to make 
a path for itself by driving the molecules of air 
towards each other and a second conductor and 
thus using them as its path. 

The ordinary electric lighting current em- 
ployed for incandescent lamps is not suflSciently 
powerful in pressure to jump across an air space, 
even if very small, although sometimes by ac- 
cident some conducting substance may come in 
the gap forming an air space separating two 
conductors, and thus help the current across. 
In such a case, however, a continuous arc would 
be formed which diifers from a stream of sparks 
jumping from one conductor to another when 
these two are connected to an apparatus which 

The A ,B C of the X Rays. 31 

is producing currents of very high potential with 
extremely rapid interruptions. 

Klectric lighting circuits for incandescent lamps 
and ordinary power are usually of comparatively 
low pressure, or voltage, generally between 50 
to 120 volts, and in some cases as high as 240 
volts. In all these systems, however, the rate 
of flow, or amperes, of current is usually large on 
account of the nature of the effects to be pro- 
duced. The voltage in any such system is kept 
constant and steady and regulated within a few 
points, but the strength in amperes of current 
will vary very considerably according to the 
requirements of the consumers. 

The electric current 'necessary to properly 
excite a Crookes vacuum tube to the degree at 
which the X rays are produced is enormously 
high in voltage, as compared with that of an 
electric light circuit. Such a current may vary 
in pressure or potential from ten thousand to 
hundreds of thousands of volts. The current 
in amperes, of such a circuit will, however, be ex- 
ceedingly low, being only a very small fraction 
of one ampere in all cases. 

It will be seen, therefore, that the actual elec- 
trical energy required to produce the X rays is 
comparatively small ; indeed, the actual amount 

32 The A B C of the X Rays. 

of energy needed to light up an ordinary i6 
candle power lamp to incandescence will be suf- 
ficient, when employed with the proper appara- 
tus, to cause excitation enough to produce abun- 
dant and powerful X rays. 

The reader will naturally inquire how this 
can be. To this we answer that it is by a trans- 
formation of the electrical energy from one form 
of potentia;l to another. This is done by means 
of certain kinds of apparatus which will presently 
be described. The construction of this appara- 
tus is based upon certain electrical phenomena, 
the laws of which have been well defined and 
availed of for the perfecting of many useful 

The best known form of apparatus and the 
one most generally used for transforming elec- 
trical energy of comparatively low voltage and 
high amperes into a current of very high voltage 
and low amperes is the Induction Coil ; frequently 
called the Ruhmkorff Coil. Before proceeding 
to describe this, we shall, for the better under- 
standing of the reader, devote a chapter to a 
brief explanation of the phenomena to which it 
owes its existence. In this explanation will also 
be involved the philosophy of another type of ex- 
citing apparatus, known as the High Frequency 

The A B C of the X Rays. TiZ 

Transformer, now coming into general use in 
connection witli X ray work. This device is 
also one of tlie class which, like the Ruhmkorff 
Coil, depends upon electrical induction for its 

A third and entirely distinct type of exciting 
apparatus embraces what are known as Static 
Machines, which, while they rely upon induction 
for the "effects produced, do not require to be 
continuously connected with a source of elec- 
trical energy, such as a' battery or an electric 
lighting circuit, but from an initial charge biiild 
up, produce and discharge electric current as 
long as they are kept in rotation. Such ma- 
chines are generally known as Holtz Machines 
and Wimshurst Influence Machines. 

All of these three classes of exciting appara- 
tus will be taken up in their order, but we shall 
first glance at the theory of the phenomena of 


34 The A B C of the X Rays. 



It is not only in connection with the apparatus 
for production of the X rays that the reader will 
find a study .of induction profitable, but also in 
connection with other electrical phenomena. 
We can scarcely lay too much stress upon the 
importance of electromagnetic induction in the 
practical electrical apparatus in use at the present 
time. It lies at the very foundation of all elec- 
tric light and power devices now in operation ; 
in fact, almost all working electrical apparatus 
depends upon inductive effects for successful 

It might naturally be supposed by those un- 
acquainted with electrical science, that, when a 
current of electricity is sent through a wire or 
other conductor, the influence of such current is 
confined to the wire or conductor and to the 
apparatus with which such conductor may be 
connected. This, however, is not the fact. The 

The A B C of the X Rays. 35 

influence of the current extends also outside of 
and away from tlie conductor over which it is 
passing, affecting other conductors to a greater 
or less extent according to their distance and 
their relative positions. 

When an electric current is passed through a 
conductor, a change ensues in its neighborhood. 
A force that was not previously present is at once 
manifested and continues to some extent as long 
as there is current passing through the conduc- 
tor. Surrounding the conductor there is an 
action which may be compared with a series of 
elastic rings expanding or collapsing, with 
changes in the current. These are called lines 
of force and are sometimes referred to as the 
magnetic whirl or magnetic field. They are 
diagrammatically represented in the sketch 
Fig. 4. 

Fig. 4. 

Magnetic lines are, of course, invisible to the 
eye and are merely ideal indications of direction 
of magnetic force or flux at any one time. In 
a stationary arrangement, with steady current. 

36 The A B C of the X Rays. 

they are stationary in space, — merely expanding 
or collapsing with changes in the current. 

In a wire observed from one end, if the cur- 
rent is away from the observer, the lines are 
directed clock-wise (as the hands of a clock 
move), around the wire, the direction being always 
understood to be the direction along which north 
polarity tends. Thus, if a bar of iron be placed 
horizontally above a conductor leading away 
from the observer and the current goes away 
along the wire, the bar exhibits a north magnetic 
pole at its end toward the right and a south pole 
toward the left. If the bar be under the wire 
the polarity is the same as to direction around 
the wire, but now the north will be directed to 
the left and xhe south to the right. The reversal 
of the current will reverse all directions. 

These lines of force may be considered to ex- 
tend out almost indefinitely and to affect con- 
ductors at great distances, but their effect is only 
appreciable at very small intervals of space. 

A simple illustration, somewhat analogous, 
may be taken to enable the reader to grasp the 
idea more clearly. 

A stone dropped into a pond of still water will 
produce a series of rings, gradually widening but 
decreasing in power as they ripple away from the 

The A B C of the X Rays. 


central cause of disturbance and finally disap- 
pearing from the vision. 

Some interesting experiments may be tried in 
illustration of this phenomena of the magnetic 
lines of force by passing through a piece of card- 
board or stiff paper, a conductor from a dynamo, 
or from a powerful battery of primary or second- 
ary cells and scattering upon the cardboard or 
paper a small quantity of fine iron filings. If the 
paper is gently tapped, say, with the end of a 
lead pencil, the filings will arrange themselves 
in circles about the wire, 
thus showing the direction 
of the lines of force (Fig. 5). 
An active conductor from a 
dynamo or sufficiently power- 
ful battery will attract to 
itself iron filings if brought 
into contact with them. 

With a small compass or delicate horizontal 
galvanometer the presence and direction of the 
lines of force around an active conductor can be 
traced. To perform this experiment, it is well to 
have apiece of stiff cardboard or thin wood through 
which the conductor is run. The compass or 
galvanometer is then placed on this table in 
different positions around the circumference of 

Fig. s. 


The A B C of the X Rays. 

tlie wire, and the needle 

Fig. 6. 

will be observed to 
deflect. On revers- 
"^ ing the polarity of 
the current the de- 
flection will be in 
the opposite direc- 
tion. Supposing the 
observer to be facing 
the upper end of the 
conductor (Fig. 6), 
the direction of the 
lines of force will be 
from left to right when the current is flowing 
from the upper, or positive, end of the conductor, 
but of course, in the opposite direction on revers- 
ing the polarity of the current. Figure 6 is a 
diagram of the deflections obtained with the 
compass as seen by an observer facing the upper 
end of the conductor. 

It should be noted in connection with this ex- 
periment that unless a powerful current is used, 
the influence of the earth's magnetism will 
probably affect the needle to a greater or less 

Let us now see what effect the magnetic lines 
of force above described have upon other con- 
ductors lying near to the active one, through 

The A B C of the X Rays. 39 

whicli a current is passing. We will place a 
separate wire, or conductor (calling it, for con- 
venience, the secondary wire), near to and par- 
allel with the active wire. The lines of force, 
which immediately spring out, cut across the 
secondary wire and cause a momentary current 
of electricity to be set up therein. This is what 
is known as Blectromagnetic Current Induction, 
and the current set up in the secondary wire is 
an induced current. 

This idea may be somewhat difficult to grasp 
in definite form, as we are dealing with manifesta- 
tions of the energy of an invisible force, but prob- 
ably a further reference to a water analogy may 
be of assistance by enabling the reader to establish 
in his mind a comparison with something visible 
and at the same time easily understood. It must 
be remembered, however, that this analogy is not 
exact, and is only used with the above idea in 
view. IvCt us place in a large vessel of water 
two corks, at a distance of several inches apart. 
If one of the corks be suddenly pushed down- 
wards, and released, a series of gradually extend- 
ing rings will form in the water and, as they 
reach the other cork, it will be affected thereby 
and will also rise and fall. These rings may be 
likened to the lines of force present around a 

40 The A B C of the X Rays. 

conductor carrying current ; the cork whicii was 
pushed down representing the conductor, and 
the energy expended in pushing it may be con- 
sidered as the equivalent of electric energy pass- 
ing through the conductor. The other cork, of 
course, represents the secondary and is affected 
by the energy set up from the first one. 

So long as the current flowing in the active 
wire remains steady, no further effect is percept- 
ible in the secondary wire, but if the continuity, 
strength or alternations of the current, or the 
relative positions of the active and secondary 
wires, be varied, there will be induced currents 
set up in the secondary wire at each variation. 
The same effect will be produced each time the 
current is made and broken through the active, 
or primary wire, and the current induced in the 
secondary wire will flow in one direction on send- 
ing current through the primary, but will flow 
in the opposite direction when current is either 
broken or reversed. 

Electro-magnetic Induction may, therefore, be 
described as an effect caused in a conductor by 
variation in a current passing through another 
and separate conductor lying parallel thereto. 

Thus far, we have used for illustration the 
terms " active " and " secondary " wires, by 

The A B C of the X Rays. 41 

whicli there may have been conveyed to the 
reader's mind the idea of two comparatively short 
pieces of wire laid side by side. This idea serves 
as an aid for the more easy comprehension of 
the broad idea of an induced current, but in 
actual practice, great lengths of wire are used, 
the mechanical arrangement of which we shall 
briefly describe in explaining the induction 

It will be of interest to the reader, however, 
to explain a little further what the result is when 
considerable lengths of wire are used to obtain 
induced currents. It is not difficult to see from 
the foregoing that, if the primary or active wire 
be coiled in layers upon a bobbin and the second- 
ary wire be coiled in layers upon and over the 
primary coil, there will be a great many lines of 
force thrown out when a current is sent through 
the primary. 

These lines of force, cutting across the second- 
ary wire, induce in all the convolutions thereof, 
a current, and this current which is induced in 
the secondary will also create lines of force, 
extending from the secondary. These inductive 
effects may be further increased by placing within 
the primary coil an iron core. 

When a current is started through the primary 

42 The A B C of the X Rays. 

coil, it develops magnetic lines in the core and 
space around it, which, in developing cut the 
turns of the coil, thereby checking the rise of 
current by a counter electromotive force. It 
therefore takes time to fully magnetize the core, 
owing to what is called its " self-induction." 
When we, on the other hand, attempt to cut off the 
current, the collapse of the magnetic lines results 
in their again cutting the coil turns and develop- 
ing an electromotive force, which tends to prevent 
the stoppage of the current. This is another 
effect of self-induction, but prolongs the original 
current and may produce a spark or arc at the 
place where the circuit is opened. 

It will be remembered that all these inductive 
effects are momentary and take place only at the 
instant of making or breaking the circuit, or on 
varying the strength or alternations of the cur- 
rent, or, on varying the relative positions of the 
coils. On breaking the current to the primary, 
all these lines of force contract, or fall back, upon 
the conductor from which they sprang, and in 
doing so, cut across the secondary again and 
induce current therein in the opposite direction, 
which is, of course, in the same direction as the 
primary current. It will be quite apparent, there- 
fore, that the effect upon breaking the circuit is 

The A B C of the X Rays. 43 

more powerful than, that upon making, owing 
to the suddenness of the magnetic changes in the 
core. If the magnetic change on making could 
be made as sudden as at the break, the inductive 
effects in the secondary would be equal in each 

In an ordinary induction coil the pressure 
and volume of the current passed through the 
primary undergoes a change. It goes into the 
primary as a current of comparatively low pres- 
sure and high volume, but the induced current 
at the terminals of the secondary will be found 
to be of comparatively high pressure and small 
volume. The desired effects are obtained from 
an induction coil by winding for the secondary, 
a number of convolutions of fine wire of definite 
size proportionate to the size of wire and number 
of convolutions of the primary. For instance, 
we might say, that to obtain at the terminals of 
the secondary, ten times the pressure passed 
through the primary, there should be at least 
ten times the number 'of convolutions in the 
secondary that there are in the primary. If the 
pressure at the secondary terminals were thus 
raised ten times, the volume of current would 
be approximately one-tenth that which passed 
through the primary, but the total of the electri- 

44 The A B C of the X Rays. 

cal energy which originally passed into the pri- 
mary would not be increased. 

Induction coils are largely used, therefore, for 
the purpose of obtaining sparks, as sparks re- 
quire high pressure, but only a small current 
passes at each spark. These sparks are of vary- 
ing length, according to the amount and size of 
wire and the number of its convolutions. A coil 
three or four inches in length, containing from 
four to six ounces of wire, will produce sparks of 
one-eighth to one-fourth of an inch in length. 

Ordinarily it is roughly calculated that one 
inch of spark could be produced for every pound 
of fine wire wound around on the secondary coil, 
but the most recent methods of winding tend to 
some modification of the rule. 

The smaller and medium sized coils are largely 
used to obtain sparks for gas lighting, for firing 
fuses and for many other useful and experi- 
mental purposes. Small coils are also used by 
physicians for medical purposes, in which case 
handles are connected by flexible wires to the 
terminals of the secondary and the patient re- 
ceives a rapid succession of shocks as the current 
is made and broken. 

The inductive effects in an induction coil are 
usually produced by a rapid making and break- 

The A B C of the X Rays. 45 

ing of the current passing through the primary. 
This, in the simple form of coil, is effected by 
means of a vibrating armature which will be 
seen in Figure 7. It consists of an iron button 
placed on the end of 
a piece of spring steel 
set in a metallic post. 
When at rest, the 
spring touches, or 
makes contact, with 
another post to which 
is attached one con- 
nection from the bat- 
tery which supplies 
current to the primary 

coil. To the first named post is attached one ter- 
minal of the primary coil. The other terminal of 
the primary coil is attached to the battery. When 
current is turned on, the electricity flows through 
the primary and immediately acts upon the iron 
core in such a way as to convert it into an electro- 
magnet. The core, thus becoming magnetic, at- 
tracts the armature and draws it away from the 
post. This breaks the continuity of the circuit, 
and, therefore, shuts off the current from the 

As no current is now passing through the 

46 The A B C of the X Rays. 

primary, the iron core loses its magnetism, and 
cannot keep the armature attracted to it. The 
armature being mounted upon a spring, flies back 
and once more touches the post. This again 
completes the circuit, just as it did before, and 
the armature is again attracted to the magnetic 
core for an instant, which once more breaks the 
circuit, and so on, thus effecting a continuous 
vibration and consequent make and break of the 
circuit as long as the battery is connected. It is 
difficult from this explanation to conceive the 
extreme rapidity with which these successive 
makes and breaks take place, but the reader has 
had, or will probably have, an opportunity of ob- 
serving it for himself. 

The A n C of the X Rays. 47 



As the reader will have gathered from the 
preceding chapter some knowledge of the funda- 
mental laws of induction and of their application 
in the form of apparatus therein described, let 
us now proceed to the consideration of some of 
of the practical points which present themselves 
upon a study of the induction coil and its parts, 
and as a whole. 

An ordinary induction coil consists of the 
following parts : 

I. The core. 

3. The primary coil. 

3. The secondary coil. 

4. The terminals of the secondary, or " spark- 

ing points." 

5. The contact breaker. 

6. The condenser. 

7. A pole changer (optional, not necessary). 
It is not within the proposed scope of this 

48 The A B C of the X Rays. 

work to give directions for constructing an in- 
duction coil, as there are many valuable books 
on this subject already published and readily 
accessible, but we will make mention of some of 
the more important points that may prove of 
practical value to the reader. 

Taking up in their order the parts making up 
such a coil, we first reach the Core, which is 
simply a cylindrical-shaped bundle of soft iron 
wires each of small cross section. These wires 
are firmly bound together so as to form, in ap- 
pearance at least, a solid piece. The core is 
sometimes soaked in shellac or paraffine to 
obtain good insulation, and sometimes it is 
given one or more outside wrappings of insulat- 
ing tape for the same purpose. In large coils 
both these methods are sometimes employed. 

Two things are very important in constructing 
the core, one being that the iron from which the 
wires are drawn is of the kind known as " soft " 
iron, and the other that the diameter of the 
wires should be quite small. Objectionable 
heating of the core may result, by reason of what 
is known as hysteresis, on account of the quali- 
ties possessed by some kinds of iron. The same 
objectionable effect would also occur, by reason 
of Foucault, or eddy, currents if a solid iron core 

The A B C of the X Rays. 49 

or wires of large diameter should be employed. 
The use of iron wires of small diameter is made 
to ensure a rapid demagnetization, as well as to 
avoid heating efEects. 

After the core is insulated, by wrapping, or by 
slipping over it an insulating tube, the primary 
coil is wound upon it. This coil -is of coarse 
copper wire, well insulated, and consists usually 
of two layers, one wound over and on top of the 
other. Bach layer is insulated from the other. 
Some makers wind the primary coil in more than 
two layers, but it is not a practice to be recom- 
mended, as the best effects cannot, generally 
speaking, be obtained in that way. 

We come now to the most important part of 
an induction coil, namely, the secondary coil. 
While the size and quantity of wire used for 
making this, together with the manner in which 
it is distributed or wound will determine the size 
and quality of the spark produced, the question 
of perfection and thoroughness of insulation will 
present itself for perhaps the foremost consider- 

This feature requires the most thorough care 
and calculation, for the high potentials at which 
the sparking effects are produced may cause the 
sparks to pierce both the primary and secondary 

50 The A B C of the X Rays. 

coils and, by breaking down the insulation tbere- 
on, render the whole coil utterly useless for the 
purpose for which it was designed. 

After the primary coil is wound it is usually 
covered with a thick tube made of many layers 
of parafi&ned paper, or with a tube consisting of 
hard rubber, micanite or some other insulating 
substance, entirely free from metallic particles. 
Upon this is commenced the winding of the 
secondary coil. The wire used for this coil is 
very small in cross section and usually has a 
cotton or silk covering. The winding should be 
done very carefully and each layer -well insulated 
with paraffine, shellac or other compound. Be- 
sides this, a sheet of paraffined paper is usually 
laid between each layer. 

The secondary of a large induction coil will 
contain many thousands of turns of this fine wire, 
and, of course, a large number of layers. There 
are wide differences of opinion in regard to the 
distribution of the turns of wire upon the second- 
ary as well as the methods of winding and the 
degree of insulation. There is also a great dif- 
ference in opinion among manufacturers as to 
the amount and size of the wire to be used on the 
secondary coil. The extent of this diiference of 
opinion may be appreciated when it is stated that 

The A B C of the X Rays. 


an induction coil recently made to give a spark 
of 12 inches weighed over 400 pounds, while the 
writer has one in constant use (made by another 
manufacturer), which, v/hile it will give heavy 
14 inch sparks, weighs only about 75 pounds. 

The ends of the wire forming the secondary 
coil are connected with two binding posts which 

Induction Coil. 

A, Base ; B, Coil ; C, Discharge Terminals ; D, Contact Breaker ; 

E, Battery Connections ; F, Pole Changer. 

are usually made to contain two rods free to slide 
therein so that the length of the spark may be 
increased or diminished within the rated limits 
of the coil. These are known as the secondary, 
or discharge, terminals. 

We now come to the contact breaker, which 

52 The A B C of the X Rays. 

is also an important part of tlie apparatus. The 
reader will recall the fact that an induced current 
in a conductor in which current is flowing occurs 
at the instant the circuit is made or broken, but 
not while current may be steadily flowing. It is 
apparent, then, that the more abrupt and com- 
plete the make and break (especially the latter), 
the more pronounced is the inductive effect ob- 

It is the aim of manufacturers, therefore, to 
make the contact breaker quick and certain in its 
action. On referring to Fig. 7, it will be seen 
that the ordinary circuit breaker consists of a flat 
spring carrying an iron armature. This spring 
is attached to a post and is normally so placed 
as to keep the armature away from the core of 
the induction coil, but is so arranged that it will 
make contact with another post that forms part 
of the battery circuit. The action of the circuit 
breaker has already been described at the end 
of the chapter preceding this one, so there will 
be no necessity of repeating it here. 

The developments of experiments with the X 
rays have demonstrated that this form of con- 
tact breaker is not the one best adapted for ob- 
taining results of the higher type, and the march 
of improvement has necessitated the adoption of 

The A B C of the X Rays. 53 

other forms, especially with, large coils. We 
propose, however, to treat of these forms in 
another chapter. 

The condenser of the ordinary form of induc- 
tion coil is usually placed in the interior of a 
hollow wooden base upon which the coil is 
mounted. The form of condenser most generally 
supplied consists of a number of sheets of tin-foil 
separated from each other by one or more thick- 
nesses of paper, which may or may not be par- 
af&ned, but preferably the latter. Bach sheet 
of tin-foil has a " lug " or end projecting out 
beyond the paper, such lugs being placed alter- 
nately at the ends. These lugs are all connected 
together at each end of the condenser, thus 
bringing each set of sheets to one conductor. 
These are then connected so as to be in shunt 
with the contact breaker. 

The office of the condenser is to accumulate 
temporarily a charge of electricity upon the rup- 
ture of the primary circuit, and when stored to 
its full capacity to discharge a current suddenly 
and quickly, thus adding to the high potential 
output of the coil. This is an exceedingly im- 
portant adjunct of an induction coil and will be 
a little more fully discussed under the separate 
chapters on Condensers. 

54 The A B C of the X Rays. 

The last item under our enumeration is the 
Pole Changer. This is not so much an item of 
importance as it is of convenience,it being intended 
to enable the operator to change the polarity 
of the primary current going into the coil and 
thus reverse the polarity of the high potential 
current that is discharged at the secondary termi- 
nals. The same result can be attained by revers- 
ing the connections of the two conductors carry- 
ing the primary current, but not so speedily or 
conveniently as with a properly arranged pole 
changer. In experiments with the X rays it is 
sometimes desirable to change the polarity of the 
current for the purpose of making certain experi- 
mental observations, although in practical work 
it is generally unnecessary after the apparatus has 
once been set up and started, as will appear later. 

It may assist the reader to more easily com- 
prehend the construction of such a coil as has 
been described by referring to the sectional draw- 
ing, Fig. 9, in which the various parts are 

Having glanced briefly at the construction and 
operation of the ordinary form of induction coil, 
let us now see what developments in the perfec- 
ion and modification of such coils have been made 
to render them more adaptable for use in X ray 

The A B C of the X Rays. 


Fig. 9. 
A, Core; B.B, Primary Coil; C, Secondary Coil; 
Terminals; E, Base. 

DD. Discharge 

work. In a subsequent cliapter we shall present 
some suggestions as to the sizes and capacities 
of induction coils required by experimenters 
and others for various degrees of practical re- 
sults expected to be attained in this interesting 

The lines upon which the most recent improve- 
ments in induction coils have been made are as 
follows : 

{a) The amount and size of wire upon the 
secondary coil, and the method of winding it ; 

(^) The insulation of the primary and second- 
ary coils ; 

{c) The contact breaker ; and 

{d) The condenser. 

56 The A B C of the X Rays. 

These items are the important ones in induction 
coils for X ray work for the following reasons : 

(i) Currents of very potential and a certain 
quality of spark are essential to obtain the best 
results in the generation of the X rays. 

(2) Induction coils are usually worked for com- 
paratively long periods of time and at high pres- 
sures for this class of experiments, thus requiring 
exceptionally high insulation. 

(3) A high number of breaks, with, certainty, 
regularity, and freedom from injurious heating, 
is desirable. 

(4) Perfection in condensers, with relation to 
their capacity, insulation and quick discharge, 
has been found to be essential. 

Taking up these points in order, we find that 
there are quite a variety of opinons as to the size 
and amount of wire to be used for the secondary 
coil as well as the methods of winding it. A 
long coil wound with very small wire will give a 
long, thin spark and demagnetizes slowly, while a 
shorter and thicker coil wound with heavier wire 
will give a shorter but a " fat " spark, and will act 
more quickly than the longer one. Combinations 
have been made of these methods, by which very 
desirable results have been obtained. 

Many coil makers wind the secondary in a 

The A B C of the X Rays. 57 

greater or less number of sections, especially in 
making large coils for sparks of 6 inches in 
length and upwards. In most coils, the second- 
ary is wound in equal turns from end to end, 
while in others the greatest number of turns are 
in the middle of the spool, gradually decreasing 
towards each end. There are other methods of 
winding, — in fact, the subject is one of such great 
detail that it would be out of the question to do 
more than make a general reference to it in this 
book, leaving the reader to consult one or more 
of the many valuable text-books on coils and 
their construction, should he desire to build a 
coil for himself. 

As we propose to treat in this chapter only of 
the practical value and uses of induction coils 
for X ray experiments, it becomes important to 
make some further mention of the question of 
insulation, — the vital part of the apparatus. Up 
to the time of Prof. Roentgen's discovery, induc- 
tion coils were not subjected, generally speaking, 
to the severe and constant use that has been de- 
manded of them in investigations of the X ray 

It was not an unusual thing, however, prior 
to that time, to puncture the insulation of a coil 
during the course of experimental work, but this 

58 The A B C of the X Rays. 

has happened mucli more frequently during the 
new class of experiments, especially in the case 
of coils made some time ago. 

The aim of the careful manufacturer at this 
day, in making induction coils for X ray work, 
is to insure the utmost degree of perfection in 
the insulation of the primary and secondary coils 
as well as to provide such a winding as will insure 
a continuous stream of fat, heavy sparks. 

There is probably no more efficacious way of 
producing the most perfect insulation of the 
primary and secondary coils than by entirely 
immersing them in oil. In this way it is practi- 
cally impossible to break down the coil, as the 
oil affords the greatest possible degree of protec- 
tion from sparking both internal and external. 

Where a coil is insulated in this way, it is 
usually placed in a box which is intended to be 
filled with oil. The primary and secondary ter- 
minals are, of course, led out to suitable binding 
posts on the exterior of the box, and the circuit 
breaker is introduced in the outside circuit. The 
writer has had such a coil in constant use many 
hours per day for several months in connection 
with X ray experiments and investigations, 
withoxit having had the slightest trouble by 
reason of the insulation. 

The A B C of the X Rays. 59 

. Tlie coil box may be provided with, means to 
allow of the oil being drawn off at any time 
should it be necessary or desirable. The box 
should also be provided with a cover which can 
be tightly closed to keep the oil from collecting 
dust or particles of metal or other substances 
that might gradually work into the layers of wire 
and cause trouble. A coil immersed in oil as 
described should be capable, with this protection, 
of practically continuous work for an indefinite 
period of time. 

6o The A B C of the X Rays. 



The Contact Breaker is a device whicli is used 
only in connection with apparatus of the Ruhm- 
korff Coil type. The ordinary form of contact 
breaker, consisting of a vibrating spring with 
armature attached has already been described in 
Chapter III. This is the form that has been 
most generally used for many years on induction 
coils, especially in the lower range of sizes, such 
as those giving sparks of six inches and less. 
The developments, however, which have been 
made, in connection with X ray experiments, 
have necessitated other forms which are much 
more desirable for this class of work. 

For the other purposes for which induction 
coils were most frequently used, it was not often 
that a large amount of current was applied to the 
primary coil, and therefore, the spark which re- 
sulted upon breaking the circuit was not espe- 
cially destructive. In working on the X ray 

The A B C of the X Rays. 6i 

experiments it is found desirable and necessary 
to use a comparatively large amount of current. 
The sudden interruption .of this current by means 
of a vibrating armature would result in a de- 
structive spark at each break of the circuit. This 
would have two effects, namely, to burn away 
the metal points at which contact is made, and to 
heat up the spring and gradually change its 
temper. Besides, the effects obtained in the 
Crookes tube would be uncertain and unsatis- 
factory in using a large coil with a contact 
breaker of this kind. 

There is also another undesirable feature of 
vibrating circuit breakers where the amount of 
current used is comparatively large, and that is, 
the irregularity of the vibrations and also their 
limited number. For the production of the X 
rays and obtaining the best results therefrom it 
is very desirable that the circuit to the primary 
coil shall be made and broken very rapidly, reg- 
ularly and with certainty. 

To obtain these results, it is now usual to 
employ a mechanical circuit breaker, operated by a 
small motor. These mechanical circuit breakers 
are made in various designs according to the 
views of different experimenters. 

62 The A B C of the X Rays. 

The important features of a contact breaker of 
tliis kind are : 

(i) That that part which operates for the mak- 
ing of the circuit shall be sufl&ciently long to 
allow the current to energize the primary coil ; 

(2) That the break shall be sufficiently long 
and sharp to insure the quickest possible break- 
ing of the circuit without drawing an arc ; 

(3) That the number of revolutions of the con- 
tact breaker shall be steady ; and 

(4) That the brushes shall bear evenly upon 
the surface. 

A. contact breaker of this kind need not be 
made so that both polarities of the current shall 
be broken. One of the rings upon which the 
brushes rest may be continuous, while the other 
may be divided into segments according to the 
number of makes and breaks desirable. 

A good form of contact breaker can be made 
by mounting two rings upon a circular slate 
base, one of the rings having several sections 
cut out for the purpose of obtaining the necessary 
breaks in the continuity of the current. These 
1 ngs may be fastened to the slate base by means 
of screws and the whole mounted either upon the 
motor shaft or upon a separate shaft fitted into 
bearings and arranged ^vith a pulley to be driven 

The A B C of the X Rays. 63 

by a motor. A contact breaker of tbis latter 
class is illustrated in Fig. 10. 

Fig. 10. 

Two bnisb-holders are mounted so tbat brushes 
may be inserted and so arranged as to bear evenly 
upon tbe surface of these two rings. If the slate 
is cut so as flush with the surface of the bro- 
ken ring between the sections a smoother running 
apparatus will be the result than if the face of the 
slate is below the surfaces of the separate sections. 

It will be found that even in contact breakers 
of this kind it is impossible to avoid the spark 
which results on breaking the current. It is 
highly desirable to eliminate this spark if practical 
means are at hand for doing it. The general 
way of accomplishing this result is by means of 
an air-blast directed against the under surface of 
the brush upon which contact is broken. 

While it is not absolutely essential to blow 
out the spdrk, the results of doing so are of great 

64 The A B C of the X Rays. 

benefit, as a greater abundance and steadier 
stream of X rays will be obtained thereby. An air 
blast is not always possible to tbe experimenter, 
however, and probably the next best thing is to 
put the circuit breaker into a small tank of water 
and operate it in that way, thus extinguishing 
the spark as soon as it is formed. Circuit break- 
ers of this kind are to be had in the market. 

It must be borne in mind that the circuit 
breaker of the induction coil is also an exceed- 
ingly important part of the exciting apparatus as 
the results depend largely upon this particular 
item. No results can be obtained- from the in- 
duction coil without making and breaking the 
circuit carrying the current to the primary coil, 
as the reader has already learned. 

The success of the experimenter in producing 
the X rays steadily and satisfactorily depends 
quite largely upon the perfection of the contact 
breaker. If it acts smoothly and regularly, and 
with certainty of break, the rays produced will 
be correspondingly abundant and steady, pro- 
vided, of course; that the source of current is of 
the requisite strength and the vacuum tube well 
and properly made. 

While it would, of course, be desirable to 
operate small coils, giving, say, from one inch to 

The A B C of the X Rays. 65 

three inch sparks, on mechanical contact break- 
ers, there is not the necessity of such perfection 
in the break as there is in operating coils giving 
sparks of four inches and upwards. 

In the smaller class of coils, for instance, those 
giving up to, let us say, three-inch sparks, it is 
not to be expected that such an advanced class of 
work can be accomplished with the X rays as 
may be done with coils of larger capacity. This 
of course is self-apparent, as the higher potential 
of the discharge of the large coils is much more 
powerful than those of smaller coils. 

It is quite possible for the experinienter to get 
good X ray results from a three or four inch coil 
using the regular form of vibrating contact 
breaker with a good example of vacuum tube, but 
even with so small a coil the results will probably 
be better if operated with mechanical contact 
breaker. Such a coil can be so used, if desired, by 
short circuiting the vibrating contact breaker and 
connecting a mechanical one to the apparatus. 
The writer has operated quite successfully an 
ordinary form of coil giving a spark of not more 
than i^ inches in this way. 

66 Ihe A B C of the X Rays. 



The name " Condenser," as applied to this 
piece of electrical apparatus, does not always 
convey to tlie lay mind a tlioroughly apprecia- 
tive comprehension of the principles of its phi- 
losophy and action. The word " condenser " is 
most frequently used and readily understood in 
connection with apparatus for condensing steam, 
which is something visible and tangible, but, 
when dealing with electricity which can neither 
be seen or handled, the idea of a condenser is 
less easily grasped by the general reader. The 
term " accumulator " is alternatively used as a 
name for a condenser of electricity, and to a cer- 
tain extent is correct, although it should not be 
confounded with the same name as applied to 
secondary, or storage, batteries. 

Perhaps a simple analogy will assist the read- 
er to comprehend more readily the theory of 
the condenser. This analogy is not by any 

The A B C of the X Rays. 67 

means a perfect one, but will probably serve as a 
basis for comparison. 

Suppose we should take a plate of glass and 
hold it over the steam issuing from the spout of 
a tea-kettle containing boiling water. That side 
of the glass touched by the steam would imme- 
diately become clouded by reason of the steam 
condensing upon it. After a short time this 
cloudiness would assume the form of innumer- 
able little globules of water and, still holding 
the glass over the steam, the condensation would 
gradually accumulate and the globules of water 
would increase in size until by their increased 
weight they would drop off (or, we might say, 
discharge), from all parts of the plate. 

Now, while electricity is intangible and has 
neither body nor weight, we shall find this idea 
of water condensation to possess a sort of rough- 
and-ready basis of comparison in the following 
brief explanation of the condenser of electricity, 
except that it should be remembered that the 
discharges of the condenser are almost incon- 
ceivably rapid. 

These condensers are usually made of sheets 
of tinfoil, separated from each other by an in- 
sulator or dielectric. The dielectrics usually 
employed are either glass, mica or paper, and 


The A B C of the X Rays. 

when the latter is the one chosen it is frequently 
coated with paraffine. The action of the con- 
denser is due to the inductive effect of the elec- 
trical charge on one sheet of tinfoil (conductor) 
upon another across the dielectric interposed 
between them, as we shall explain a little more 

Let us suppose that we have two sheets of tin- 
foil A and B (Fig. ii), and between them a 
dielectric C, which for the purpose of the illus- 
tration, may be a pane of glass. 

Now if the conductor B be connected with the 
c positive pole of a source of electric 

_]^/^ energy and the conductor A with 
the earth, the conductor B will be- 
come positively electrified and will 
act by induction across the glass 
and upon the conductor A and 
send a positive current to the 
earth, leaving a negative charge 
upon that face of A that is nearest to B. As 
two opposite polarities attract each other, this 
negative charge of A will attract a greater posi- 
tive charge upon the nearest face of B, and the 
nearer these conductors are brought together, 
the greater the attraction for each other, which 
results in a greater accumulation or condensa- 

FlG. II. 

The A B C of the X Rays. 69 

tion of electricity upon tlieir respective surfaces. 
It will be seen, therefore, tliat the capacity for 
condensation of such a conductor is greatly in- 
creased as it is brought nearer to another con- 
ductor oppositely electrified. 

Condensers may be made in many ways, but 
for all ordinary purposes, such as for use with 
RuhmkorfE coils, they are made up of sheets of 
tin-foil (conductor), and paraffined paper, (dielec- 
tric), laid alternately, the tinfoil being much 
smaller than the paper to avoid leakage around 
the edges. Bach sheet of tinfoil has a project- 
ing lug and these are placed so as to project at 
one end alternately, as seen in the sketch. Fig. 
12. When all the 
sheets are laid, fin- 
ishing with a dielec- 
tric top and bottom, 
the whole is bound Fig. 12. 

together tightly to further increase the capacity. 
Ordinarily the entire block is soaked in melted 
paraffine. The projecting lugs at each end are 
connected together so as to form two solid ter- 

The effect of this arrangement of the con- 
ductors and dielectric is to make practically two 
large conductors of an area equal to the aggre- 


The A B C of the X Rays. 

gated area of tlie sheets of tinfoil used in making 
the condenser. When thus made, a condenser 
will have a very considerable capacity and may 
be charged with a greater amount of energy than 
if its capacity were less, either by reason of a 
smaller area of conductors or by increasing the 
distance between each of the conductors. 

The above arrangement is intended for con- 
densers of comparatively low potentials. If a 
condenser for a very high potential is desired, 
the sheets of tinfoil should be smaller in relation 
to the dielectric, and they are laid alternately as 
before, but without lugs or other connections be- 
tween the respective conductors. 
Connections are made only with 
the upper and lower conductors. 
The danger of leakage in a con- 
denser so constructed is greater 
than in the other form. 

The best known and original 
form of condenser is the Leyden 
jar (Fig. 13), so called from the 
town of Leyden, where it was 
invented. It consists of a wide- 
necked bottle coated with tinfoil 
Fig. 13. inside and out about two-thirds 

of its length. In a dry, well-shellacked cork 

The A B C of the X Rays. 71 

fitting into the neck is inserted a brass rod, the 
lower end of which carries a, small chain mak- 
ing contact with the tinfoil in the inside of the 
jar. The upper end of the rod is surmounted 
by a brass ball. This form of condenser may- 
be charged from an induction coil by connecting 
the ball and outside coating in series with one of 
the secondary terminals of the coil. By apply- 
ing one terminal of a discharger with insulated 
handles (Fig. 14) to the outside 
coating and approaching the other 
terminal to the ball the accumu- 
lated charge will be discharged 

with a detonating spark. 

There are a very large number 
of interesting experiments that 
may be performed with this and 
other forms of condenser, but as 
we are only considering this form fi^. 14. 
of apparatus from the standpoint of its use in 
connection with exciting apparattis for the pro- 
duction of the X rays, we shall not stop to en- 
large upon other experiments, especially as they 
are given in detail in most of the standard text 
books on physics. 

" It is merely intended in this chaptei to convey 
to the reader a general . idea of the theory and 

72 The A B C of the X Rays. 

use of the condenser, and to give a brief descrip- 
tion of the method of its construction. There 
remains, therefore, only a brief mention of its 
sphere of usefulness in connection with the 
apparatus under consideration. 

As used with apparatus of the induction coil 
type, the condenser is connected in shunt with 
the primary coil. 

Now, when an electric current is sent through 
the primary coil ^from the batteries or other 
source of energy, we have seen that a current is 
induced in all the turns of the secondary coil. 
Besides this, the primary, when broken by the 
contact breaker, also tends to induce current in 
itself. This is known as a self-induction dis- 
charge. This self-induction discharge would 
spark across the circuit breaker if it were not for 
the condenser, and the break of the current 
would be less sudden. The extra current is 
accumulated by the condenser at the break of 
llie circuit and is immediately discharged back 
into the primary with lightning-like rapidity, with 
the result of very greatly increasing the potential 
of the current induced in the secondary coil. 

It should be understood that the accumulation 
of charge in the condenser is fully what the terrri 
implies. It is an accumulation of charge and 

The A B C of the X Rays. 73 

potential while the primary coil is discharging, 
and if the original current had a potential of 
eight volts, the accumulation in the condenser 
might be ten or twelve times as great. Thus it 
will be seen that the effect of the whole of this 
accumulation being rapidly and suddenly dis- 
charged into the primary coil is to produce very 
powerful inductive effects. 

To recapitulate, we may say briefly that in 
connection with an induction coil the condenser 
has two objects, one being to cut down the spark- 
ing at the circuit breaker and the other, by its 
sudden and total discharge into the primary, to 
greatly increase the strength of the induced 
current in the secondary coil. 

Ordinarily, condensers furnished with induc- 
tion coils have a stated capacity which is un- 
changeable, but others are specially made with 
reference to the use of either fractional parts or 
the whole of their capacity. These are useful to 
the investigator of X ray phenomena who 
desires to procure refinements of experimenta- 
tion, but they are not absolutely necessary. 
Further reference will be made to the subject of 
adjustable condensers in the chapter on Manipu- 

74 The A B C of the X Rays. 



In a preceding chapter we have learned that 
the induction coil is an apparatus which, by rea- 
son of inductive effects, will transform a current 
of low potential and high amperes into a current 
of exceedingly high potential and low amperes. 
The primary current in this case may be a direct 
or continuous one, the inductive effects taking 
place upon the making and breaking of the cir- 
cuit by mechanical means. 

A reverse transformation of the current could 
be obtained with the same coil if a current of very 
high potential and low amperes were led into the 
terminals of the secondary. In this case an in- 
terrupted current of low potential and compara- 
tively high amperes could be obtained at the 
primary terminals. 

This kind of transformation is practically what 
is done in systems of electric lighting by means 
of alternating currents. In such a system an 

The A B C of the X Rays. 75 

electric current of high, potential is generated at 
a central station and sent through conductors to 
the points where it is used. At these points 
coils, or, as they are termed, " transformers " are 
placed. The high potential current is sent into 
the secondary of a transformer and from the 
primary terminals a current of low potential 
with large ampere capacity is taken for electric 
lighting or power. 

The transformation is effected by induction, as 
in the Ruhmkorff coil, but in dealing with alter- 
nating currents the inductive effects are not ob- 
tained by a mechanical interruption or breaking 
of the circuit, but by continuous changes of di- 
rection, or alternations, of the current. 

It will be remembered that in the chapter on 
Induction it is stated that as long as the primary 
current is steady and continuous no inductive 
effects are noticeable, but if the continuity, 
strength or alternations of such current be varied 
a current will be induced in the surrounding 
coils. Now, the current from a battery connected 
with the primary of an induction coil (Ruhm- 
korff) is continuous and flows in one direction, 
and therefore its continuity or strength must be 
varied by mechanical means, but the alternating 
current, as its name indicates, is not continuous. 

76 The A B C of the X Rays. 

but travels first in one direction and then in the 

Thus it will be seen that the alternating cur- 
rent possesses from its very nature the necessary 
quality to cause (without mechanical interrup- 
tion), inductive effects in apparatus suitably con- 
structed with reference to its employment. It 
may be here noted that the mechanical con- 
struction of transformers differs somewhat from 
that of the ordinary form of induction coil. We 
shall not attempt, however, to enter into the de- 
tails of such construction, as it would require too 
much space and is not within the scope of this 

We shall confine ourselves, therefore, to such 
points relating to alternating currents as will 
enable the reader to comprehend the action of 
High Frequency Apparatus used for production 
of the X rays. 

As alternating currents are produced in the 
first place by mechanical means, that is to say, 
by dynamo-electric machines actuated by steam 
engines or other motive power, it necessarily 
follows that the alternations of the current must 
have some degree of regularity. Diagramatic- 
ally, an alternating current is represented as 
shown in Fig. 15. The line A B is the zero line. 

The A B C of the X Rays. 77 

The current starts at the line in one direction 
and rises to its maximum potential, dying down 
again to zero, then starting again in the opposite 
direction and dying down once more to zero, and 
so on. 

Fig. 15. 

Bach of the waves represents one alternation 
either in a positive or negative direction, and two 
of these waves represent a complete double rever- 
sal, or cycle. 

The time occupied by two alternations, or a 
complete cycle, is called a period, and the number 
of periods per second is known as th.e. frequency. 
In the greater number of alternating current 
dynamos in use at this day the frequency varies 
between 60 and 130 cycles per second, or from 
7200 to 15,600 alternations per minute. It will 
be seen, therefore, that comparing the inductive 
effects in an induction coil, actuated by a con- 
tinuous current broken by mechanical means 
and similar effects produced in a coil through 
which an alternating current is flowing, the 
results in the former cannot, by reason of the 

78 The A B C of the X Rays. 

natural limits of mechanical circuit breakers, be 
as rapid, continuous and powerful as in the 
latter. When we add to the alternating coil, or 
transformer, one or more condensers, it will 
readily be appreciated that the alternating cur- 
rent transformer, or High Frequency type of 
exciting apparatus, is indeed powerful in pro- 
ducing discharges of high potential, and with 
regularity and very great frequency. 

Briefly described, a set of high frequency 
apparatus for use with vacuum tubes for the 
generation of the X rays consists of two coils, or 
transformers, with one or more condensers, two 
discharge terminals and a spark gap. The latter 
also consists of two terminals or metallic rods 
each terminated by a metal ball. As commer- 
cially manufactured at this date the two trans- 
formers and a condenser are usually placed in a 
box which may be filled with oil for high insu- 
lating purposes, and the discharge terminals and 
spark gap are located on the exterior of the box. 
Such an arrangement, together with a small 
motor air blast (the use of which will be presently 
explained) is shown in Fig. i6. 


















The A B C of the X Rays. 

fiinSiiig PoA 

ffoifntcfun Sonri 


High Freyuen ^ 

~Gfy>'r/{ Cap ^ 

Fio. 17. 

The A B C of the X Rays. 8i 

A diagram of tlie arrangement and connections 
of these transformers, condenser, discharge 
terminals and spark gap is given in Fig. 17. On 
referring to this diagram it will be seen that the 
primary of the low frequency transformer, or 
coil, is connected directly to mains from an alter- 
nating current circuit, and that the terminal 
wires of the secondary of this transformer are 
extended and are wound so as -to form the pri- 
mary coil of the high frequency transformer, 
while the condenser is in shunt with the last 
named primary and the spark gap forms a break 
in its continuity. The discharge terminals are 
the terminals of the secondary of the high 
frequency transformer. 

Let us now see what the action of this set of 
apparatus is, bearing in mind that each of the 
transformers is really an induction coil, the name 
" transformer " being usually 'given when such a 
coil is designed for work upon an alternating 

When this set of apparatus is connected with 
the circuit and current turned on, it passes 
through the primary of the low frequency trans- 
former, and, by reason of the alternations, cur- 
rents are induced in the secondary. As the 
secondary coil consists of a great many turns of 

82 The A B C of the X Rays. 

comparatively fine wire, the current induced 
therein is of vastly higher potential than the 
inducing current. Let us assume that the pri- 
mary, or inducing, current has a pressure of 52 
volts, we may find that the induced current in 
the secondary has a potential equal to 7500 

As we have observed from the diagram. Fig. 
1 7, the secondary of this low frequency trans- 
former becomes the primary of the high frequency 
transformer. We might for a moment consider 
the latter as an independent induction coil and 
the low frequency transformer as a source of cur- 
rent (similar to a battery) from which the pri- 
mary is to be excited. Indeed, it is such in reality. 
It will be apparent, therefore, that with an 
energizing current such as will flow through the 
primary of the high frequency transformer (say 
7500 volts), the ind'uctive effects in its secondary, 
wound with still finer wire, will be to produce an 
induced current of exceedingly high potential at 
the secondary terminals. 

The sole purpose of the low frequency trans- 
former (giving, say, 7500 volts at its secondary) 
is to charge the condenser, its secondary terminals 
being connected thereto. When the condenser 
is so charged as to leap the spark gap and pass 

The A B C of the X Rays. 83 

through the few turns of primary of the high 
frequency transformer, such discharge is very 
rapid, oscillating at very high frequency. The 
potential of such discharge does not rise above 
the 7500 volts, in fact it is generally less than 

If the secondary of the high frequency trans- 
former has twenty times as many turns as its 
primary, the potential is twenty times greater, or 
150,000 volts, less a considerable loss due chiefly 
to the distance of the coils from each other. A 
five-inch spark at the secondary terminals of the 
high frequency transformer would represent a 
potential of 50,000 to 75,000 volts. 

It must be remembered that the figures above 
given as a ratio of potential are not exact, but 
are intended only as an approximation and for 
purposes of comparison and elucidation. 

The reader will now naturally inquire why one 
transformer should be designated as of low fre- 
quency and the other of high frequency. 

As we have stated above, the frequency of an 
alternating current has reference to the number 
of complete cycles or double reversals, per second. 
The inductive effects in the low frequency trans- 
former occur only on each alternation of the 
original current, once as the wave begins, and 

84 The A B C of the X Rays. 

again when it dies down. Thus, if such current 
had a frequency of 65 cycles per second, there 
would be set up in the low frequency transformer 
260 inductive effects in the same period of time. 
If there were no condenser in the circuit of the 
low frequency transformer (which, it will be re- 
membered, is the primary of the high frequency 
transformer), the same number of effects would 
take place in the latter ; but the discharges of 
the condenser are even more frequent than the 
original alternations, thus multiplying the induc- 
tive effects in number and making this latter 
transformer one of correspondingly high fre- 

The function of the spark gap is to force the 
condenser to charge to a high potential before 
discharging through the primary coil of the high 
frequency transformer. As we have stated above, 
this discharge is oscillatory and occurs with very 
high frequency. Low potential arcing over the 
spark gap must, however, be prevented or the 
condenser would not charge. This is secured by 
an air blast on the gap or by special construction 
and proportional arrangement of parts ■ without 
the air blast. 

If the spark can be extinguished as quickly as 
it is formed, the break or rupture of the circuit 

'The A B C of the X Rays. 85 

will be mucli more positive and sharp than by- 
allowing the spark to weaken and die out natu^ 
rally. Various methods have been devised, there- 
fore, to extinguish the spark, chief among them 
being the electro-magnetic blow-out and the air- 
blast. The latter is the more simple method of 
the two, being usually accomplished by means 
of a small motor with fan attachment to which is 
af&xed a pipe terminating in a small nozzle so 
placed that the jet of air will be directed between 
the two terminals of the spark gap. 

Although, by means of the air-blast, the spark 
is extinguished in an exceedingly small fraction 
of a second after it is formed, the extinguish- 
ment cannot be perceived by the eye when the 
apparatus is in operation on account of their ex- 
treme rapidity of succession. When we consider 
that these sparks may jump across this small gap 
with a frequency of perhaps 500 or 600 per 
second, it will be evident that it would be 
quite out of the question to perceive the forming 
and extinguishing of an individual spark. In 
action, therefore, there appears to be a torrent of 
sparks constantly jumping across this small gap. 

To close our description of this apparatus some 
mention must be made of the discharge terminals 
from the secondary of the frequency transformer. 


The A B C of the X Rays'. 

It is here that the cumulative and entire induc- 
tive effect is obtained. Between these terminals, 
tremendously powerful sparks and discharges 
are obtained, and it is with these terminals 
that the Crookes tube is to be connected when 
the apparatus is used in X ray investigations. 
These terminals are usually in the form of 
brass standards carrying discharge rods, each 
terminated at one end by metal spheres and at 
the other end by rubber insulating handles. 

Regulator for Thomson High Frequency set. 

Some forms of this apparatus as commercially 
supplied have a small regulating coil by means 
of which the amount of current admitted to the 
low frequency transformer may be governed. 

The A B C of the X Rays. 87 

This is useful in adjusting the apparatus so that 
more or less powerful effects may be obtained 
according to the requirements or peculiarities of 
the Crookes tubes in use or the necessities of the 
particular case in hand. 

Although the description of this particular 
type of apparatus may make it appear to be some- 
what intricate and involved, its operation is 
reasonably simple with a small amount of experi- 
ence. The accessories are few, and it has one 
good feature to recommend it where alternating 
current is available and that is, it may usually 
be connected direct to the circuit and there are 
no batteries to look after. Indeed, this type of 
apparatus could not be operated by batteries ex- 
cept through the medium of costly and unusual 

The High Frequency apparatus is very power- 
ful in generating the X rays when used in con- 
junction with a good vacuum tube. The type 
of tube that seems to be best suited for these dis- 
charges is a double focus tube, for the reason 
that in this form there are three electrodes, two of 
which can be connected to the two terminals of the 
apparatus, and the bombardment of the cathodic 
stream takes place upon the third electrode, thus 
producing the X rays. It should be borne in 

88 The A B C of the X Rays. 

mind that as the high frequency set depends 
upon an alternating current for excitation, each 
of the two terminals is alternately positive and 
negative, but the high frequency with which 
these changes occur results in almost a constant 
cathodic stream bombarded upon both sides of the 
third electrode. 

There is one thing that should be noted in 
respect of high frequency apparatus and that is, 
that although it will give a reasonably sharp 
definition of objects radiographed or examined 
by the fluoroscope, the intense sharpness that 
can be obtained by a static machine is absent in 
this case. The reason is chiefly that in the case 
of the high frequency coil the discharges are 
oscillatory in character, and cause a slight diffusion 
of the X rays, while from a static machine the 
discharges are in one direction and show a clearer 

A close approximation to the sharp definition 
of the static machine may be obtained by using 
with the high frequency coil a diaphragm con- 
sisting of a sheet of metal with a hole about i to. 
i^ inches in diameter cut in its centre. This 
is placed in front of and about one inch away 
from the tube, covering all of it except imme- 
diately in front of the third, or bombarded elec- 

The A B C of the X Rays. 89 

trode, and thus cutting off the diffused or scat- 
tering rays. Such an arrangement need only 
be used when it is desired to obtain especially 
sharp definition of some particular part of an 
object to be radiographed or examined. It is not 
generally necessary when the object is near the 
surface that is nearest the sensitized plate or the 
fluoroscopic screen. 

go The A B C of the X Rays. 



In discussing induction coils and transformers 
we have been dealing with electricity in motion, 
usually known as dynamic or current electricity^ 
but in the type of machines now to be considered 
we shall find what is termed static electricity or 
electricity at rest. In the former state the cur- 
rent may be considered as usefully manageable, 
while in the latter state it is less so, manifesting 
itself by attractions and repulsions and violent 

So far as the employment of the electric cur- 
rent is concerned in producing the desired effects 
in induction coils, transformers and static 
machines, such effects are all the resiilts of the 
phenomena of induction. In the two first-named 
•types of apparatus, however, the induction is the 
direct result of the variation of ciirrents flowing 
in adjacent conductors, while in static machines 
the inductive effects are due to the attraction 
and repulsion of opposite and like electricities. 

The A S C of the X Rays. 91 

The two first-named types of apparatus require 
a constant supply of electric current to ensure 
their action, while a static machine needs only 
an initial charge of electricity, which is generally 
present in a self-charging machine, or may be 
imparted as the result of friction in some form. 
When this initial charge exists or has been 
given, the active parts of the machine are set in 
motion and from the small charge there is quickly 
accumulated a greater one which will result in a 
constant discharge as long as the machine is kept 
in action. 

It is well known that if a stick of sealing-wax, 
a rod of glass, a piece of ivory and many other 
substances (insulators in their nature) are rubbed 
with a piece of cat fur, flannel or silk, they 
acquire a new property and will attract light 
bodies, such as feathers, paper, gold leaf, etc. 
In this condition they are said to be electrified, 
and there is upon that part of the surface rubbed 
a static charge of electricity of either positive or 
negative kind, according to the nature of the 
substance and the article used as a rubber. 

When light bodies are thus attracted to an 
electrified body they remain a short time in con- 
tact and are then repelled. They do not merely 
fall away but are actually repelled for the reason 

92 The A B C of the X Rays. 

tbat, during the time of contact, they have 
acquired a condition of electrification of like 
kind, and according to the law as laid down by 
eminent scientists, two similarly electrified 
bodies repel each other, while two oppositely 
electrified bodies attract each other. 

It will be noted that in the example just given 
one body becomes electrified by contact with an- 
other body already electrified. This condition 
may also be produced by induction where the two 
bodies are not brought in contact with each other. 

Suppose we insulate a glass ball upon a glass 
rod and place near it a metallic cylinder, also 
mounted upon a glass insulating rod, as shown 
in Fig. 1 8, and then rub the ball with silk. The 
ball will become electrified positively. It will 
/r ^>^, then be found that 

'*'' ^" +y the cylinder has al- 

so become electrified 
by induction by rea- 
son of the presence 
of the ball. The 
positive electricity 
has, however, been 
repelled to the further end of the cylinder while 
the ball has attracted towards itself negative 
electricity at that end of the tube nearest to it. 

The A B C of the X Rays. 93 

These effects- will take place even if a sheet of 
glass were interposed between the glass ball and 
cylinder and would also be found if they were at 
some distance apart. The inductive effects would 
cease if the ball were removed altogether and no 
charge would be found upon the cylinder, although 
the ball might still be electrified. 

If the cylinder should be connected to earth 
while the ball were in its proximity the positive 
charge would be repelled into the ground and 
none would be found on the -further end of the 
cylinder, but a negative charge would still be 
attracted to the end nearest the ball. 

Electricity in this state is said to be " bound '' 
and " free." It is said to be " bound " when it 
is attracted and apparently neutralized by a 
charge of the opposite kind near to it, and " free" 
when it is not in the immediate presence of a 
charge of the opposite sign. For instance, in 
the illustration above given, the negative (or — ) 
charge induced upon the end of the insulated 
cylinder is " bound " by the attraction of the 
positive (or +) electrification of the insulated ball, 
while the + charge repelled to the end of the cylin- 
der furthest from the ball is " free," and will 
flow to the earth if a conductor be offered. The 
— charge on the cylinder, which is " bound " by 


The A B C of the X RayS. 

the + attraction of the ball will remain " bound " 
even if a conductor, or path, be offered for it to 
go to earth. 

It may be superfluous as regards some of our 
readers but helpful to others in more readily 
comprehending the philoso- 
phy of a Static Machine, to 
describe an instrument de- 
vised by Volta for procuring 
by induction a number of 
charges of electricity from a 
small initial charge. This 
instrument is called the 
FIG. 19. " Blectrophorus," and con- 

sists of a cake of resinotis material and a disk of 
metal, or wood covered with tinfoil, in which is 
af&xed an insulating handle of glass (Fig. 19). 
If the cake be rubbed with a piece of dry, warm 
woolen cloth or a piece of fur, it becomes nega- 
tively electrified. If then the disk is placed upon 
the surface of the cake, this 
negative electrification acts 
inductively upon the metallic 
cover and attracts a positive 

. Q, charge to the under side and 

repels a negative charge to 

the upper surface. This is 

Fig. 20. shown diagrammatically in 


+ ++ + + + + + 

The A B C of the X Rays. 95 

Fig. 20. In this case the + charge on the under 
side of the disk is bound, while the — charge on 
the upper surface is free. 

If a conductor be offered to the free charge it 
will immediately escape to earth, leaving only 
the bound charge on the under side of the disk, 
as shown in Fig. 21. On lift- 
ing the disk the + charge will 
distribute itself over both the 
upper and lower surfaces of 

the disk. If the finger be ^ 

brought near the disk a dis- \_^ _^ + + ' + + 2 
charge of this positive electri- j" ' ? 

fication will manifest itself by fig. 21. 

causing a spark to pass, and the operation can 
be repeated several times with like results with- 
out renewing the charge upon the resinous cake. 
All these effects can be repeated indefinitely 
upon rubbing the cake as often as it loses its 

It will be quite evident, therefore, that if some 
mechanical means for performing these opera- 
tions were devised there might be obtained a 
rapid and continuous succession of electrical 
discharges. As a matter of fact, quite a number 
of machines have been devised for the purpose 
of producing this result, since Volta first made 


The A B C of the X Rays. 

fhe Blectrophorus in 1775, and while they are 
all of great interest as steps towards greater per- 
fection, we shall confine ourselves to a brief 
description of those which have survived as 
the most efficient and generally useful, namely, 
the Holtz and Wimshurst machines. 

Fjg. 22. 

The Holtz Influence Machine, in its best 
known and simplest form, consists of two glass 
plates one of which is held by its edges in a 
fixed position and the other is mounted upon a 
spindle which is fixed in a standard at the back 

The A B C of the X Rays. 97 

of tlie machine. Upon the shaft of this latter 
plate there is a pulley attached, by means of 
which the plate may be revolved through the 
medium of a belt and driving pulley, as shown 
in Fig. 22. 

Two holes are cut in the fixed plate at points 
diametrically opposite to each other. These are 
usually termed " windows," and a piece of var- 
nished paper is fastened on the back of the plate 
near each of these windows, one piece being 
affixed above one of the windows, and a piece 
below the other one. These are termed " arma- 
tures." Bach armature is provided with a 
narrow tongue which projects through the win- 
dows towards the movable plate, and pointed in 
a direction opposite to that in which this plate 
revolves, but so arranged as not to touch it. 

In front of the movable plate a stationary 
conductor is placed, extending diagonally across 
the plate, at each end of which there is a " comb " 
consisting of a number of metal points directed 
towards the plate. 

So far, all these parts are directly concerned 
in the action of the machine, but as it is desired 
to collect and utilize the electrical discharges 
some provision must be made therefor. On each 
side and in front of the movable plate are placed 

98 The A B C of the X Rays. 

horizontal metallic combs supported upon in- 
sulated holders. These combs are joined to brass 
rods terminating in brass balls and to these brass 
rods Leyden jar condensers may be attached. 
The discharges of the machine take place be- 
tween the two brass balls. 

When the character of the discharge desired 
is simply the " silent " or brush discharge, the 
Leyden jars are not connected, but when sparks 
of high potential are requisite the jars are used 
to obtain the result of the accumulated charge 

To start the machine it is generally necessary 
to impart to it a small initial charge, which may 
be done by electrifying one of the armatures 
with a rubbed glass rod, or any other substance 
previously electrified. Some machines are so 
constructed that a rubber plate conveniently 
placed will impart this initial charge to one of 
the armatures on being rubbed or struck with a 
piece of fur. 

As soon as the initial charge has been given, 
the movable plate is rotated and the machine 
will very rapidly build up its charge, giving out 
a stream of sparks between the discharge 
terminals in a few seconds after the rotation is 

The A B C of the X Rays. 99 

Bearing in mind the earlier portion of this 
chapter as to the inductive effects of charged 
bodies and the consequent attractions and repul- 
sions, the reader will find it comparatively easy 
to understand the action of the machine. 

Let us first see vs^hat the result of the initial 
charge is without rotating the plate. We will 
suppose that we have imparted a -|- charge to the 
armature on the right hand side. This charge 
acts inductively across the glass and air and neg- 
atively electrifies the comb on the lower end of 
the diagonal conductor, repelling a + charge to 
the other end of this conductor. This, in turn, 
discharges + electricity upon the front surface 
of the plate which acts inductively upon the other 
armature, charging that part opposite the comb 
with — electricity and repelling a + charge into 
the part farthest away, namely, the tongue. The 
tongues at the left and right are, therefore, dis- 
charging upon the back of the revolving plate posi- 
tively and negatively electrified air, and the combs 
attached to the diagonal conductor are also dis- 
charging similarly upon the front of this plate. 

On turning the plate around, the + charge 
discharged on the back by the left hand tongue 
comes over to the right side, and being free is 
discharged into the armature to which we origi- 

ioo The A B C of the X Rays. 

nally gave the -f charge, thereby increasing the 
strength of its charge and causing it to act more 
strongly than before. The — charge induced in 
the left hand armature now reacts on the upper 
comb causing a more powerful discharge of -j- 
electricity from its point and strengthening the 
charge drawn through the diagonal conductor. 

It will be seen, therefore, that both the front 
and back surfaces of the movable plate are both 
positively and negatively electrified, the front 
from the induction originating from the arma- 
tures and the back from the inductive effects 
caused by the electrification of the front surface. 
It will also be noted that the -\- and — electric- 
ities upon the back of the plate serve to strength- 
en and maintain the original inducing power of 
the two armatures, while the surplus -f and — 
electricities induced upon the front surface of 
the plate are, after the machine has arrived at 
its maximum charge, collected by the two hori- 
zontal metallic combs and discharged across the 
terminals in front of the machine. 

There are various modifications of the original 
Holtz machine made at this date, the one most 
generally known being that originated by Toe- 
pier. This consists chiefly in afiixing small 
metallic buttons on the front of the revolving 



iiiliTiiMhimin — 

Made with Single-focus Tube and Wimshurst Machine, by Mr. H. C. Ogden, Middletown, N Y. 

The A B C of the X Rays. loi 

plate and providing small wire brushes attaclied 
to tlie combs. Metallic arms are affixed to the 
armatures and so arranged as to touch the but- 
tons lightly as they pass. Feeble charges exist 
in these buttons, and they therefore become car- 
riers, discharging as they come in contact with 
the brushes attached to the combs. In other 
words, the buttons and brushes are provided so 
that excessively feeble charges if they exist may 
yet be transferred as if the machine were in full 
work and since the rows of points could not do 
that, — actual contact is essential. Such feeble 
charges always exist. Hence the machine is 
self-charging if in perfect order. 

Holtz machines are very susceptible to moist- 
ure and dust ; and are usually enclosed in cases 
for protection therefrom. Ordinarily an artifi- 
cial drying agent such as chloride of calcium or 
sulphuric acid and pumice is introduced into the 
case to absorb any moisture that may arise and 
to ensure a perfectly dry atmosphere surround- 
ing the machine. Such a machine should be 
kept in as dry a place as possible to obtain the 
most advantageous results from it in operation. 

It is quite un.usual in these days to make 
these machines with a single rotating plate, ex- 
cept for the more simple kinds of experiments, 

io'2 The A B C of the X Rays. 

Macllines for X ray investigations are usually 
made with two or more revolving plates, some 
being made with as many as 24 plates. These, 
however, are quite elaborate and costly, and are 
only employed, generally speaking, by men of 
scientific attainments desiring to follow special 
lines of experiment and investigation. It may 
als6 be noted that the plates may be made of 
hard rubber atid need not necessarily be of glass. 

The commercial Holtz machines giving from 
4 to 10 inch sparks are very desirable forms of 
apparatus for X ray investigations. Their dis- 
charges are not oscillatory, but in one direction, 
and, with single focus Crookes vacuum tubes, 
are capable of giving very fine results either for 
fltioroscopic examinations or in making radio- 
graphs. In the latter there is perhaps as little 
conflisioH of outlines as it is possible at this time 
to obtain. Such machines also have the advan- 
tage of beitlg independent of batteries or electric 
lig'ht circuits. As we have already seen, their 
dischafge's afe created by rotating the plates. 

The reader must bear in mind, however, that 
we eannot get from nature something and give 
fl-othing in return. The rotation of a few plates, 
of themselves, irequire only a little expenditure 
of power, but when we generate electrical energy 

The A B C of the X Rays. 103 

thereby we must be prepared to give something 
in exchange therefor. This something is power, 
and while this does not amount to ver)' niuch 
for a machine that will produce sparks of small 
length, it becomes an item for consideration 
when the sparking length reaches 6 to 8 inches. 
The reader is recommended to provide some 
motive power should he decide upon purchasing 
a large Holtz machine, for the reason chiefly 
that much greater regularity of operation can be 
thereby obtained and the results will be corre- 
spondingly better. 

The Wimshurst Machine. — This is an ap- 
paratus of more recent invention than the orig- 
inal form of Holtz machine. It is shown in Fig. 
23, and as will be seen, differs somewhat in ar- 
rangement from the Holtz machine. 

In this machine there are at least two revolv- 
ing plates but no stationary plate. It may be 
made with as many pairs of plates as are found 
desirable or necessary for the purpose, and the 
plates may be either glass or rubber. 

Upon each plate is placed a number of strips 
of tinfoil or thin metal, usually called " sectors " 
which serve two purposes, namely, as carriers 
and armatures. There is an uninsulated diag- 

I04 The A B C of the X Rays. 

onal metal piece in front of the face of each plate, 
each piece bearing at its end a soft metallic 
brush. A standard at each side bears collector 
combs on front and back, connected with the dis- 
charge terminals in front. 

Fig. 23. 

The action of this machine is, generally speak- 
ing, somewhat similar to that of the Holtz ma- 
chine, that is to say, an initial charge is im- 

The A B C of the X Rays. los 

parted and the inductive effects by attraction 
and repulsion are obtained. 

Each, of the carriers is touched by one of the 
brushes as it comes opposite the charged arma- 
ture of the other plate, and the action of the ma- 
chine is similai- to that of the Holtz-Toepler pre- 
viously described, the foil and brushes being for 
the same purpose as the buttons and brushes in 
that machine. 

yo6 The A B C of the X Rays. 



The reader whose acquaintance with electrical 
apparatus may be slight, or whose only knowl- 
edge as to the X ray phenomena may have been 
gleaned from the newspapers, might very natu- 
rally suppose that the name Crookes tube would 
indicate a piece of apparatus of some specific and 
definite shape and size. 

Such, however, is not the fact. The name of 
" Crookes," as applied to an exhausted glass tube 
of any size or shape, either with or without elec- 
trodes, indicates the degree of vacuum which 
the tube should possess. The eminent English 
scientist, William Crookes, after extensive re- 
searches upon the subject of electrical discharges 
in high vacua, gave to the world, in 1879, the valu- 
able results of his studies and experiments in 
this direction, and developed a series of tubes of 
a certain degree of exhaustion, by means of which 
certain interesting phenomena, until then un- 
known, might be observed. 

The A B C of the X Rays. 107 

A Geissler, Crookes, or other vacuum tube may 
be generally described as consisting of a glass 
chamber of spherical, cylindrical or other shape, 
into which is sealed, by means of platinum wires, 
two or more metallic electrodes which serve to 
carry the electric current into and out of the bulb. 
These tubes can also be made without electrodes 
being carried into the interior, tinfoil, or other 
metallic electrodes being fastened to the oiitside 
or placed in close proximity thereto. These 
latter types of tubes are rarely used, however, 
by reason of their lower degree of ef&ciency and 
a greater liability to puncture. 

Prior to the researches and discoveries of 
Crookes, experiments had, for many years, been 
made with electrical discharges in vacuum tubes, 
or in tubes containing various gases, by Geissler 
and others, but the vacua in these tubes were 
not of the high degree attained in the Crookes 
tubes. The phenomena observed in these low 
vacuum tubes were very striking and beautiful, 
and in fact, Geissler tubes are largely used at 
this day in demonstrating certain features of 
electrical discharges under such conditions. 
Low vacuum tubes, are chiefly remarkable for 
the striae and beautiful luminous effects which 
they exhibit when an electrical discharge is 

tcS The A B C of the X Rays. 

passed througli them. These effects are due to 
various causes which may be, respectively, the 
degree of vacuum, the various kinds of gases or 
liquids contained in the tubes, or the different 
kinds of glass of which they are made, or a com- 
bination of two or more of these items. 

The luminous effects produced in Geissler and 
other comparatively low vacuum tubes by elec- 
trical discharges are said to be due to the violent 
agitation of the molecules of the gases therein, 
and by the constant collision of such molecules 
with each other during the period of such dis- 
charges. Ordinarily, the molecules in a low 
vacuum tube have a tendency to move in straight 
lines, but their average " Free path " is very 
short and they collide with each other, producing 
the characteristic cloudy luminescence. A 
vacuum tube of this kind has, by comparison 
with a Crookes tube, a high internal pressure. 
With greater rarefaction of the tube, and a con- 
sequent lowering of the pressure, or, in other 
words, a higher degree of its vacuum, the " Free 
path " of each molecule becomes longer and they 
move in straight lines with less mutual interfer- 
ence than before. 

The cloudy, luminous effects observed in the 
low vacuum tubes are not present in the Crookes 

The A B C of the X Rays. 109 

tube. Instead, the whole interior of the latter 
tube is clear, but the glass of the tube itself as- 
sumes a beautiful fluorescence which it retains 
as long as the electrical discharge is continued. 
The fluorescence is the most brilliant at that 
part of the tube opposite the cathode electrode. 
This fluorescence of the glass is never present in 
a Geissler or other low vacuum tube ; indeed it 
is one of the infallible signs by which a Crookes 
tube may be recognized. In many Crookes tubes 
there will also be observed a continuous succes- 
sion of brighter fluorescent flashes along the 
glass in that part of the tube nearest the cathode 

The color of the fluorescence of the glass will 
vary according to the kind of glass used for 
making the tube. In some kinds of hard, Ger- 
man and other glass, the fluorescence will be of 
a bright apple-green color, while in other kinds 
the color will vary from a lighter green to a 
canary color. Some kinds of lime glass have a 
mottled fluorescence of uncertain color, while 
lead glass fluoresces a beautiful robin's-egg blue. 

Tubes of the types designed by Mr. Crookes 
have been quite extensively used by experi- 
menters and in institutions of learning for many 
years for demonstrating the numerous and 

no The A B C of the X Rays. 

interesting phenomena incident to electrical dis- 
charges in very high vacua, but it was not until 
after the discovery by Professor Roentgen of the 
X rays that much special attention was paid to 
the fluorescence of the glass of the tube. In the 

Fig. 24. 

announcement of his discovery it was stated that 
the X rays apparently proceeded most strongly 
from that part of the Crookes tube opposite to 
the cathode electrode, the point of greatest fluo- 
rescence of the glass. 

It was found by others who .took up this line 
of experiment that the greatest amount of X rays 

The A B C of the X Rays. 1 1 1 

was obtained from this point when using Crookes 
tubes of the type in which the electrodes were so 
placed that the rays therefrom were directed 
against the glass of the tube itself. Two of 
these types of tubes are illustrated in Figs. 24 
and 25. 

Fig. 25. 

The illustrations • show two of the types of 
Crookes tubes that were used to obtain the 
earlier results in X ray experiments, indeed the 
pear-shaped type is still successfully employed 

112 The A B C of the X Rays. 

for the same purpose, althougli the more ad- 
vanced investigators have begun to realize the 
greater advantages of the focussing tubes which 
we shall presently discuss. 

On referring to Figs. 24 and 25, it will be 
seen that the electrodes consist of flat metallic 
disks. These are usually made of aluminum. 
It will also be seen that both of these disks face 
some point of the glass tube, without interfering 
with each other, and that one of the disks is 
larger than the other. 

In practice, when either of these types of tube 
is used, the larger disk is so connected as to be- 
come the cathode, and the smaller disk the anode. 
When the current is turned on, the cathodic 
steam caused by the electrical discharge strikes 
the glass at the part opposite the negative elec- 
trode, and, if the tube is properly made, the 
characteristic fluorescence will at once appear, 
but strongest and best on the glass opposite the 
cathode. It is also at this point that the X rays 
are produced and may be observed with a 
fluoroscope or by means of a sensitized photo- 
graphic plate. Little, if any, manifestations of 
the presence of the X rays are noticeable at the 
point opposite the anode electrode. 

Although Crookes tubes of the types illus- 


Mummif ed Hand of an Egyptian Princess, obtained near the Tomfac 

of the Kings, Thebes, iSgz. 

The hand is believed to be between 3,000 and 4,000 years old. 

Half-tone : Etched, using Carbutt Process plates. 


of Mummified hand on preceding- page, made in the Laboratory of 

the Keystone Dry Plate Works, on a Carbutt X-Ray plate, by 

John Carbutt. 

Half-tone : Etched," using Carbutt Process plates. 

The A B C of the X Rays. 113 

trated in Figs. 24 and 25 have been, and still 
are, successfully used in the production of the 
X rays, they do not represent the best type of 
tube for this purpose. In the first place there, 
is great diffusion of the rays, originating as they 
do from the cathodic stream proceeding from a 
flat metal disk having no point of focus. This 
diffusion gives rise to a more or less blurred 
shadow of objects as seen in the fluoroscope or 
as shown in a radiograph made upon a sen- 
sitized plate. This lack of sharp outline, how- 
ever, is less in degree if the X rays produced 
by this type of tube proceed from electrical' dis- 
charges generated by a static machine. The 
reason of this is that these discharges are usually 
in one direction and have less of the oscillatory 
character accompanying the discharges from ap- 
paratus of the induction coil type. 

The diffusion and consequent lack in sharp- 
ness of outline may be overcome to some extent 
by the use of a diaphragm, as we have already 
explained. While greater definition may be 
thus obtained, the use of a diaphragm neces- 
sitates a longer exposure in taking a radio- 

There is another undesirable feature of the 
type of Crookes tubes with which we are now 

1 1 4 The A B C of the X Rays. 

dealing, namely, that type in which the X rays 
proceed from the point of the glass directly bom- 
barded by the stream of cathodic rays. This 
feature is the heating of the glass at that point. 

It may not be generally understood that the 
stream of cathode rays generates considerable 
heat at the point where it strikes, but such is 
the fact ; and the more powerful the source of 
this stream, the greater the heat. The writer 
has seen, in a tube of the type illustrated in Fig. 
25, ten inches in length, a spot as large as a five 
cent piece, opposite the cathode, become almost 
red hot in about three minutes under steady ex- 
citation of the tube with a 9 inch spark coil. 

This heating is much greater in a new tube 
than in one that has been in use, at intervals, 
for some time. 

Indeed, it is customary for those acquainted 
with these facts to " cure " tubes of this type 
before putting them to constant use. The 
" curing " of a tube is accomplished by running it 
on a comparatively light discharge for a few 
seconds at a time, with intervals of a similar or 
greater length of time, and continuing this treat- 
ment for an hour or two. After being " cured " 
in this manner the tube will develop less heat 
than before at the point above mentioned, and is 

The A B C of the X Rays. 115 

then ready for practical use in X ray work. It 
should be added that there is also considerable 
heat developed at that point of the glass that is 
opposite the anode electrode, but not of so great 
a degree as at the cathode. This is also modi- 
fied when the tube is cured, as the vacuum is 
then higher. 

Many Crookes tubes of this kind are spoiled 
when new by the heating and melting or crack- 
ing of the glass soon after the current is applied, 
and even tubes that have been cured will ulti- 
mately be destroyed in this way, although, by 
means of careful manipulation, they have previ- 
ously had a long and useful life. 

And now, having described the general forms 
of the Crookes tubes that were so successfully 
used for the earlier demonstrations of the X ray 
phenomena, let us see what the natural progres- 
sion of ideas on the subject has brought forth 
with the view of perfecting our knowledge of 
this new and wonderful discovery. Before pro- 
ceeding, however, it may be well to note that 
some of the newer forms of Crookes tubes are 
known under names that are employed to dis- 
tinguish them, either commercially, or as the in- 
vention or design of individuals. It should be 
borne in mind that, at this day, all forms of 


The A B C of the X Rays. 

vacuum tubes tliat are capable of use for the pro- 
duction of the X rays may be properly classified 
under the generic title of " Crookes Vacuum 
tubes," while the later forms, such as " double 
focus " and " single focus," tubes which were 
especially designed for purposes that Mr. Crookes 
could not have had in mind, may be considered 
as species. 

It will be remembered by most of the readers 
of this book that Mr. Edison was among the 
earliest of the X ray experimenters in America 
after the announcement from abroad of Professor 
Roentgen's discovery. Taking up- the subject 
in his characteristic way, Mr. Edison made an 
endless variety of forms of Crookes tubes, and, 
after numerous experiments, finally decided 
upon the adoption of a form substantially as 
illustrated in Pig. 26. 



Fig. 26. 

It will be seen that in the Edison tube the two 

The A B C of the X Rays. 1 1 7 

electrodes are opposite eacli other, each electrode 
being inclined at an angle. Under the influence 
of oscillating discharges each electrode will alter- 
nately become anode and cathode and the X rays 
may be observed at that part of the tube against 
which the cathodic streams are directed. This 
tube was a great advance for X ray investiga- 
tions over the forms just described. The rays 
are abundant, and comparatively clear definition 
of opaque objects interposed between the tube 
and a fluorescent screen or sensitized plate can 
be obtained. 

This form of tube has been very successfully 
used by Mr. Bdison in his investigations of the 
X ray phenomena for several months past. It 
is also worthy of note that it was the form of 
tube employed in the four weeks' public exhibi- 
tion given by him in May, 1896, at the National 
Electrical Exposition in New York City, when 
many thousands of persons saw for the first 
time the manifestations of the X ray phenom- 

The disadvantage of diffusion of the source of 
the X rays proceeding from the earlier forms of 
Crookes tubes became apparent as soon as the 
investigation of the phenomena brought to light 
some of the possibilities incident to the applica- 

ii8 The A B C of the X Rays. 

tion of Professor Roentgen's discovery. There 
seemed to be but one way in which sharp out- 
lines cotild be obtained, and that was to bring 
the rays to a focus. There appeared to be con- 
siderable difficulty in the way of accomplishing 
this object, as the X rays could not be refracted 
or condensed by lenses such as are used to focus 
rays of light. 

From the time that the discovery of the X 
rays was first published, probably no one in the 
United States has been a closer or raore enthu- 
siastic student of the subject, not only from 
the theoretical but also from the practical point 
of view, than Professor Elihu Thomson. After 
making a somewhat exhaustive series of experi- 
ments with the types of Crookes tubes that were 
known in the earliest history of the subject, it 
became a settled conviction in his mind that the 
most efficient type of vacuum tube for practical 
use in X ray experiments was one capable of 
focussing the cathodic rays and projecting them, 
so focussed, in straight lines. 

As early as January, 1896, Prof essor Thomson 
used in his X ray experiments a standard form 
of Crookes tube, as shown in Fig. 27. This tube 
had a piece of platinum in the centre and a con- 
cave cathode of about one inch in diameter at one 

The A B C of the X Rays. 


Fig. 27. 

end. The rays came to a focus on the platinum 
and X rays were produced in abundance and 
sharp definition was obtained by him from this 
focussing tube. This same form of standard 
Crookes tube was also used early in 1896 by a 
few other experimenters, among whom were Mr. 
Shallenberger and Mr. Scribner. In April, 1896, 
it was announced in the technical journals, that 
a tube of substantially the same design had been 
brought out in England for X ray investigations 
and used successfully. This form is known 
commercially as the single focus tube. 

In the light of investigations made by him with 
Ruhmkorff coils as exciters, Professor Thomson 

1 20 The A B C of the X Rays. 

had arrived at the conclusion that the nature of 
the discharges in a Crookes tube was to some 
extent oscillatory. In designing a vacuum tube 

Fig. 28. 

which would focus the rays, it appeared desirable, 
therefore, that the tube should be one that would 
be capable of doing so under discharges of such 

The A B C of the X Rays. 121 

a nature, and at the same time be useful as a 
focussing tube under discharges of a uni-direc- 
tional nature, such as those derived from a Holtz 
or Wimshurst machine. The ultimate result of 
his deductions and experiments was the double 
focus tube illustrated in Fig. 28. 

This form was intended as one capable of uni- 
versal application to the discharges of all known 
kinds of exciting apparatus. 

It will be seen, on referring to the illustration, 
that there are two concave electrodes placed dia- 
metrically opposite to each other and between 
them another electrode, wedge-shaped. The 
latter electrode is made of platinum and the other 
two are made of aluminum. 

In using the double focus tube in connection 
with a Ruhmkorff coil, the terminals of the two 
cups are connected together and then connected 
with the cathode, or negative terminal, of the coil. 
The terminal of the wedge-shaped electrode is con- 
nected with the positive, or anode. When the cur- 
rent from the coil is discharged through the tube, 
the two cathodic streams bombard the platinum 
wedge, and the two cathode electrodes being cup- 
shaped, the cathodic rays are focussed to a point 
and strike the anode upon each side. The X rays 
are thus produced in great abundance and are 

122 The A B C of the X Rays. 

projected from tlie anode electrode in straight 
lines normal to its surface. 

If the double focus tube be used upon a higli 
frequency coil actuated by an alternating current, 
one of the two cups is connected to one terminal 
of the coil and the other cup to the other termi- 
nal, for the reason that, as the current alternates 
rapidly, the polarities of the coil terminals are 
also alternating with the same rapidity. Thus, 
each one of the cups is first anode and then ca- 
thode, and so on, each of these changes or alter- 
nations taking place in a very small fraction of 
a second. The results are very similar to those 
obtained by using the tube in connection with a 
Ruhmkorff coil. 

When used with high frequency apparatus the 
terminal of the platinum wedge may be grounded. 
This is not necessary, however, although the 
results may be somewhat increased by doing so. 

To use the double focus tube on a static 
machine only one of the cups and the wedge- 
shaped electrode are connected to the terminals 
of the machine. As we have already stated, the 
discharges from such a machine are seldom os- 
cillatory, but generally in one direction, and no 
useful result with small output machines would 
follow from making connections so as to have 

The A B C of the X Rays. 123 

two catliodes in this case ; indeed it would prob- 
ably be disadvantageous as tbe force of the ca- 
thodic stream would be split up and the X rays 
produced might be of less power. On the whole, 
the double focus tube is perhaps not as desirable 
for use with the static machine of small output 
as the single focus tube. 

When employed in connection with the other 
types of exciting apparatus, however, the double 
focus vacuum tube is all that can be desired. It 
gives sharp definition of opaque objects examined 
with the fluoroscope, and also enables the operator 
to make good radiographs of such objects upon 
photographic plates. With exciting apparatus 
of some power the X rays are produced so 
abundantly and with such power that the ex- 
posure required for radiographs is reduced to a 
low point, and fluoroscopic examinations may be 
made with much certainty and satisfaction. 

The single focus tube, as illustrated in the 
technical journals, in April, 1896, was very 
similar to the standard form illustrated in Fig. 
27, and consisted of a glass globe containing an 
aluminum cup for the cathode electrode and a 
disk of platinum set at an angle of about 45 de- 
grees for the anode electrode. The cathodic rays 
focus upon this anode disk and are projected 

1- The A B C of the X Rays 

iorward therefrom, as in the case of the double 
focus tube. 

It is not absolutely necessary to use a round 
disk of platintim for the anode of a single focus 
tube ; a square, triangular or star-shaped piece 
is frequently employed by manufacturers in this 
country as illustrated in Fig. 29. The single 

Fig. 29. 

focus tube when well made is a most excellent one 
for the production of the X rays. It also gives 
sharp definition of opaque objects examined or 
radiographed, and is capable of use for most of 
the advanced class of work now carried on. This 
type of tube is, however, better adapted for use 
with static machines and the smaller and medium 
sizes of coils rather than in connection with the 
high frequency apparatus or large coils. 

This chapter would not be complete without 
some mention of (i) the natural rise of the 

The A B C of the X Rays. 125 

vacuum in a Crookes tube wlien in constant use : 

(2) the effect thereof ; and (3) what can be done 
to bring back the vacuum to a condition of use- 

It is well known to those who have made ex- 
tensive use of Crookes tubes in X ray investi- 
gations that, after some time, the vacuum will 
rise so high that the electrical discharges from 
the exciting apparatus cannot be forced through 
the tube. The fluorescent and the X ray effects 
will cease, and, usuallj', the inside of the tube 
will have a blackened dirty appearance. This 
is due to the disintegration of the platinum wires 
or the platinum electrode and the depositing of 
the particles on the wall of the tube. These 
particles are driven off under the influence of the 
electrical discharges and, upon cooling, they, as 
well as the wires and electrode, absorb or condense 
gradually what little residual gas is left in the 
tube, thus creating almost an absolute vacuum. 

As an electrical discharge will not pass freely 
in a vacuum of this kind, the tube thus becomes 
useless for the generation of the X rays. If the 
outside of a tube in this condition be heated by 
the flame of a Bunsen burner or spirit lamp the 
particles of platinum will throw out the air they 
have absorbed and by causing the discharges to 

126 The A B C of the X Rays. 

take place while the tube is in this heated con- 
dition, the characteristic fluorescence and the X 
rays will again appear. 

This, after all, is but a temporary expedient, 
and is only successfully applicable a limited 

Fig. 30. 

number of times. With each time of heating 
the subsequent usefulness of the tube becomes 
less and less, until finally no further work can 

The A B C of the X Rays. 127 

be obtained from it. Tlie only recourse wben 
that time arrives is to have it re-exhausted by the 

Professor Blihu Thomson has provided in his 
double focus tube for this difficulty. He attaches 
to the stem (see Fig. 30), a small auxiliary tube 
containing a quantity of a chemical substance 
which has the property of discharging vapor 
upon being heated. The Thomson tube when 
completed in the factory is exhausted to the 
proper degree of vacuum so as to give fluores- 
cence and produce the X rays, just in the ordi- 
nary way. When, in course of time, after some 
use, the degree of vacuum has risen so high that 
the X rays are no longer given out, a gentle heat 
is applied to the small auxiliary tube and imme- 
diately sufficient vapor is driven off to bring the 
vacuum down to the proper degree, and the X 
rays once more manifest themselves in abun- 

This operation is repeated as often as neces- 
sary, and in this way the usefulness of the tube 
is prolonged for a great length of time. If the 
chemical substance in the auxiliary tube should 
not outlive the main tube itself, another small 
tube with a new supply can at any time be 

128 The A B C of the X Rays. 

The auxiliary vacuum adjuster tube is also 
affixed to single focus tubes, and may also be 
placed on all otber types of Crookes tubes. It 
is, however, advisable, if one should desire to put it 
upon an especially valuable tube, to have it done 
by those who are expert, as it is quite a delicate 
operation, to say nothing of the increased diffi- 
culties attending the subsequent re-exhaustion. 

The A B C of the X Rays. 129 



As stated in a previous chapter, the Fluoro 
scope owes its existence to the fact that certain 
crystalline chemical salts possess the peculiar 
property of exhibiting fluorescence when brought 
within the sphere of influence of the X rays, 
There are several chemical salts that possess 
this peculiarity, but only two kinds have up to 
this time been found most generally useful. 
One of these two is platino-cyanide of barium 
and the other is tungstate of calcium. The crys- 
tals known as platino-cyanide of potassium also 
possess this fluorescent property, but from the 
fact that they are apt to deliquesce, or turn to 
liquid, under the influence of the ordinary atmos- 
phere, this particular crystal has not found favor 
for use in this connection. 

For some time after the discovery by Pro- 
fessor Roentgen, of the X ray phenomena, pla- 
tino-cyanide of barium was the only chemical 

1 3D 

The A B C of the X Rays. 

crystal generally known to exhibit ffuorescence. 
Mr. Bdison, however, in one of his characteristic 
searches, discovered that tungstate of calcium ex- 
hibited as much, if not a greater degree of fluores- 
cence. The calcium tungstate also possessed 
the merit of costing only about one-third of the 
other salt, in fact, since its fluorescent properties 
were first announced by Mr. Bdison, the price of 
tungstate of calcium has been reduced in cost so 
that it can be purchased at about one-ninth of 
the price of platino-cyanide of barium. 

To Mr. Edison should also be ascribed the 
credit of making the practical device known as 
the Fluoroscope, by means of which these fluo- 
rescent salts can be 
practically used by 
even the most inex- 
perienced person for 
the purpose of ob- 
serving the influence 
of the X rays there- 
upon. A brief des- 
cription of this device 
will enable the reader 
to fully appreciate its 
practical value. 

Fig. 31. 

Leaving out of consideration for the moment 

The A B C of the X Rays. 131 

the question of size, the fluoroscope may be de- 
scribed as a tapering box provided at one end 
with an opening edged around with some soft 
dark-colored material into which the upper part 
of the face will fit closely and exclude light 
(Fig. 31). At the other end of the box is placed 
what is known as a " fluorescence screen." This 
is made either of cardboard, wood, thin alumi- 
num, thin hard-rubber or any other substance 
through which the X rays can easily penetrate. 
The inner side of this screen is coated with the 
crystals of either platino-cyanide of barium or 
tungstate of calcium, most generally the latter. 
The whole apparatus is made as light-tight as 
possible, with the exception of the opening at 
the top which is intended for the observer to look 

The most important feature of the fluoroscope 
is the screen, of course. If the crystals are not 
evenly distributed, that is to say, if they are 
thickly distributed in one place and thinly in 
another, the fluorescence of the screen will be 
unequal in brightness. A great deal therefore 
depends upon an equal distribution of the crys- 
tals and upon getting the coating of a proper 
thickness. Naturally, the quality of the fluo- 
rescent material is also an important item, but 

133 The A B C of the X Rays. 

that which, is now generally to be had in the 
market is found to be of sujG&ciently good quality 
to give fair results. 

As stated in the first chapter of this book, the 
crystals upon the screen will exhibit fluorescence 
if the X rays are not interfered with by opaque 
objects placed between the Crookes tube and the 

In making examinations of objects through 
the medium of the X rays, the observer places 
the fluoroscope to the eyes so as to shut out any 
outside light. If there are X rays present, the 
screen will glow with the characteristic fluores- 
cence, resembling somewhat a ground glass win- 
dow pane as seen at night with a light at some 
little distance away and behind it. 

Let us suppose that the observer wishes to ex- 
amine some object, as for instance, a lead pencil. 
The pencil should be placed against the outside 
of the screen, and as close to it as possible. Upon 
looking through the eye-piece there will seem to 
be only a very thin straight line, and if the rays 
are particularly strong and penetrating, nothing 
else will be seen. This is the graphite inside 
the wood. The rays penetrate the wood instantly 
and thus cast no shadow of it upon the screen. 
As the graphite in the pencil is more opaque to 

The A B C of the X Rays. 


tlie X rays they cannot pass througli it so well, 
and for that reason cannot cause tHe fluorescence 
of the crystals upon that part of the screen im- 
mediately in front of the graphite of the pencil, 
consequently a dark line representing the graph- 
ite is seen upon the screen. 

If the object placed in front of the fluorescent 
screen should be a leather pocket-book with coins 

Fig. 33. 

therein the X rays would in like manner pene- 
trate the leather, and reach the fluorescent crys- 
tals and that portion of the screen which was 
immediately behind the leather would fluoresce 
just as if nothing were interposed. The coins 
being made of metal wotild, however, intercept 
the rays so that they could not strike the fluores- 

134 The A B C of the X Rays. 

cent crystals on the screen and thus the coins 
would be outlined and appear black to the ob- 
server looking at the screen. The same effect 
would be produced by placing the hand in front 
of the screen, and pressed tightly on the outside 
of the screen, and in front of the tube. The 
rays would pass through the flesh, blood, sinews 
and muscles, but the bones being opaque, their 
shadows would be cast upon the screen. 

Of course, there is no permanent shadow left 
upon the screen by placing any objects in front 
of it as above mentioned. As soon as the genera- 
tion of X rays ceases, there is, of coiirse, nothing 
to cause the crystals to exhibit fluorescence, and 
the screen will, therefore, become dark. It is 
evident, therefore, that the fluoroscope is a device 
which can be used indefinitely if properly taken 
care of. 

No sure rule can be laid down as to how close 
it is necessary to bring the fluoroscope to the 
Crookes tube which is being excited. Where 
the apparatus is very powerful and the tube is 
generating a great abundance of X rays, the 
fluoroscope will enable a person standing ten or 
twelve feet away from the Crookes tube to see 
the bones in his hand clearly. 

This, however, is what might be considered 

The A B C of the X Rays. 135 

an extreme case. Ordinarily, the observation is 
made within one or two feet from the vacuum 
tube. With a very good double focus tube 
strongly excited the writer has known a screen 
to exhibit fluorescence 50 feet away, with two 
doors and a wooden partition interposed. This 
is also quite exceptional and should not be ex- 
pected in ordinary practice. 

136 The A B C of the X Rays. 



It does not need a very far-reaching view into 
■ lie future to warrant the statement that a physi- 
cian will soon be somewhat behind the times if 
he does not either possess apparatus for making 
radiographs and examinations by means of X 
rays or have ready access to such apparatus. 
In considering the acquisition of this apparatus 
the foremost consideration which presents itself is 
how to obtain the electric current by which it is 
to be operated. Physicians are usually very busy 
persons, whose time is fully occupied, not only 
during the day but frequently during the night. 
They are not exempt from professional demands 
even on Sundays or holidays. Consequently 
they dread, as a rule, the care and time which is 
necessary to operate a set of batteries and keep 
them in order. 

Not only does this important question present 
itself to physicians but also to experimenters 
who desire to make a study of the X ray phe- 

The A B C of the X Rays. 


nomena. For the benefit of physicians and ex- 
perimenters, it is proposed to classify the dif- 
ferent kinds of apparatus which may be used for 
the exciting of Crookes tubes and state opposite 
each item how such apparatus may be success- 
fully used. The following is a tabulated state- 
ment of this kind : — 

Exciting Apparatus. Source of Operation. 

Ruhmkorff Coil. 

Blectric Light Cir- 
cuit, direct current. 
Batteries, either pri- 
mary or storage. 

High Frequency Appa- 
ratus (or X ray Trans- 

Alternating Current 

Static Machine 
(Holtz or Wimshurst.) 

1. Hand or foot power. 

2. Blectric motor. 

3. Water motor. 

4. Gas motor. 

1 38 The A B C of the X Rays. 

Static Machines. — From an examination of 
the table above referred to it will be seen that the 
only type of apparatus that does not absolutely 
require electric current either from a lighting 
circuit or from batteries is the Static machine. 
This machine generates its own electricity when 
the plates are revolved. The revolution of the 
plates may be accomplished either by hand or by 
a small motor of any kind that is sufficiently 
powerful for the purpose. 

Usually static machines are manufactured in 
such manner, that they may be operated either 
by hand power or by the attachment thereto of 
a small motor, for which purpose pulleys are 
usually supplied. Of course, where it is practi- 
cable, it is much more desirable and convenient 
to revolve the plates by means of a motor, chiefly 
by reason of the greater degree of regularity 
obtained thereby, but when there are no con- 
veniences to operate such a motor the X rays 
may be successfully produced and results ob- 
tained if operated by hand. 

If an electric light circuit is available, either 
alternating or direct, a small motor capable of 
giving from one-eighth to one-half horse power 
can be applied to the ordinary commercial static 
machine and used for operating it. Such motors 

The A JB C of the X Rays. 139 

are usually obtainable from any electric light 
company operating a station. In operating a 
static macbine, no batteries, rheostats or other 
accessories are needed. Indeed, this is by all 
means the most simple and convenient manner 
of producing the X rays for the usual objects 

High Frequency Coils. — Taking up next 
the High Frequency apparatus (or X ray trans- 
former) we note from the table above named, that 
the only method of operating this is by means of 
an alternating current. This apparatus is very 
easy of manipulation, and, as it is capable of use 
directly upon a lighting circuit, with the mini- 
mum of accessories, it is an ideal type of excit- 
ing apparatus where alternating current can be 

There are to-day a very large number of cities 
and towns in which there are central stations 
supplying an electric lighting current by the 
alternating system. In some cases the voltage 
of the circuit may be from 50 to 52 volts, and in 
other cases from 100 to 104 volts. As a rule, 
the high frequency apparatus that is on the 
market is so made by the various manufacturers 
that it is available for any of these circuits. It 

1 40 The A B C of the X Rays. 

is generally necessary for the purcliaser to merely 
specify the voltage and frequency of the current, 
which information can be readily obtained at the 
station supplying such current. In many of the 
largest cities of the United States there is a serv- 
ice of direct, or continuous, current, but no alter- 
nating current circuit, and inasmuch as the high 
frequency apparatus is one of the most powerful 
of all the X ray exciting apparatus, many per- 
sons will desire to use this type of apparatus 
where only direct current is available. In such 
cases it is quite possible to procure from some 
of the manufacturers of electrical apparatus mo- 
tor-transformers by means of which the direct 
current can be transformed into an alternating 
current and so applied and used with the high 
frequency apparatus. 

The motor-transformer is a very simple ma- 
chine, consisting merely of a small direct current 
motor, having upon one end of the armature two 
collector rings making contact with brushes, to 
which are attached two wires. When set up, 
two conductors are run from the direct current 
circuit to the motor, and two wires carrying an 
alternating current run from the motor through 
a rheostat or regulating coil to the high fre- 
quency apparatus. 

The A B C of the X Rays. 141 

This apparatus cannot be operated by bat- 
teries except by a very complicated arrangement 
and witli great trouble and at a very great expense. 
Tbe only methods in which it can be worked is 
in accordance with those described above, and 
always by an alternating current. 

RUHMKORPF CoiljS. — As to the operation of 
Ruhmkorff coils, it will be seen by referring 
once more to the table that they can be operated 
satisfactorily only in two ways, either from an 
electric light circuit, direct current, or by bat- 

If no electric light circuit is available for 
the purpose, there are several kinds of bat- 
teries that may be used, namely : (i) Storage 
Batteries, or, (2) Primary Batteries, such as 
Bunsen, Grove, Fuller, Grenet, or the Bdison- 

Taking up the above in their order, storage 
batteries are undoubtedly the best for the pur- 
pose, but, of course, the user must have some 
means of getting them recharged from a direct 
current circuit. 

This is quite simple and convenient if it hap- 
pens that the mains of such a circuit are conven- 
ient, as the batteries can be so arranged as to be 

142 The A B C of the X Rays. 

constantly connected and the charging current 
may be applied by turning a switch. 

Of all the primary batteries enumerated, the 
Bdison-Lalande is by all means the best, as it 
has a very large ampere capacity and does not 
deteriorate by polarization to any appreciable ex- 
tent when not in use. While the voltage given 
by each cell of the Edison-Lalande battery is not 
quite half that given by the other types of pri- 
mary battery above named, its large ampere 
output and non-polarization qualities make it very 
desirable for this class of work, when neither 
storage batteries nor direct current circuit is 

It is, of course, quite feasible to operate a 
Ruhmkorff Coil directly upon an electric light- 
ing circuit, if current from such source can be 
had, and the user does not desire to use batteries 
of any kind. 

In such a case the quantity and potential of the 
current passed through the primary of a Ruhm- 
korff coil may be regulated either by a rheostat 
or a bank of lamps or may be obtained of the right 
potential and quantity by an auxiliary apparatus 
known as a rotary transformer, or motor-dynamo. 
The rheostat may be one of the ordinary kind, and 
the bank of lamps may consist of a number of 

The A B C of the X Rays. 143 

lamps arranged upon a board, tte regulation in 
tlie latter case being accomplislied by the turning 
on or off of a greater or less number of lamps. 
The rotary transformer consists of a motor tak- 
ing current from the circuit and driving a small 
dynamo which generates the proper current for 
the coil. 

While all three of these methods are somewhat 
uneconomical, many will prefer any one of them 
to batteries. Of these three methods, the simplest 
is probably the one which employs the ordinary 
rheostat as its use simply involves the shifting 
of a handle to regulate the amount of current 

Although many experimenters object to batter- 
ies, it should be understood that there are good 
reasons for the use of storage batteries to actuate 
Ruhmkorff coils, even when a direct current cir- 
cuit can be had. On the whole they are probably 
more economical and have a degree of regularity 
in potential that cannot always be relied on with 
such a circuit. They can also be arranged, as 
above stated, in permanent connection with the 
mains and in such a case will require a mini- 
mum of attention. 

With such an arrangement, or with a rheostat 
or bank of lamps and direct current electric light 

1 44 T^^ A B C of the X Rays. 

ing circuit available, the ttouglit and attention 
of the operator may be concentrated on his obser- 
vations and experiments with X rays, and the ex- 
tra mental labor incident to side considerations 
as to the source of current may be avoided. The 
local electric light company is taking care of that 
part, and all that the operator need do to obtain 
current is to turn a switch. 

There are and will be, however, a large num- 
ber of cases where the experimenter will be re- 
moved from any source of electric current supply 
or power. In such cases there are only two types 
of exciting apparatus that can be used, namely, 
the induction xoil or the static machine. The 
latter can be operated by hand, and we shall not 
need, therefore, to give any thought to the 
question of a supply of electric current, as the 
machine contains its own source of electrical 

Where there is no direct current electric light- 
ing circuit convenient, storage batteries are, of 
course, out of the question, as they can only be 
charged from that kind of current. If, therefore, 
the experimenter wishes to use an induction coil 
and there is no regular electric lighting circuit 
available, the only source of energy will be that 
which can be obtained from primary batteries. 

The A B C of the X Rays. 145 

Any of tlie types of primary battery employing 
bicliromate solution, or the Bdison-Lalande cells, 
will be found suitable for this purpose. It is ad- 
visable to provide cells of ample size in order 
that a large ampere discharge can be obtained. 
We shall offer a few suggestions in a later 
chapter as to the .number of cells required for 

ordinary coils. 

146 The A B C of the X Rays. 



The many questions constaritly arising in 
connection with the manipulation of the appa- 
ratus used for production of the X rays require 
for their successful solution careful observation 
and thought. To many of our readers even the 
simplest methods may be entirely unknown. It 
will, therefore, probably be best to begin with a 
plain statement of the manner of arranging the 
apparatus to produce a radiograph or to make 
visual observations by means of the fluoro- 

Let us therefore commence with what may be 
considered an amateur's set of apparatus, which 
may consist of, say, an induction coil giving a 
spark, of one inch and upwards. A coil of this 
size will probably have the regular form of contact 
breaker, namely, the vibrating armature already 
described. The condenser of such coil is usually 
contained in the hollow base upon which the coil 

The A B C of the X Rays. 147 

A substantial, roomy table is always preferable 
for this work. The operator had best be pro- 
vided with a stand or holder for his Crookestube. 
An adjustable stand, that will enable the experi- 
menter to place the tube in almost any position 
is the best for this purpose. 

There are many good ones to be had in the 
market. The coil is placed a little distance 
away from the front of the table, the tube be- 
ing placed near to the front of the table within 
easy reach. The batteries may be located under 
the table and the wires brought up to the coil. 
It is always advisable to have a switch in the cir- 
cuit of one of the battery wires leading to the 
coil, so that the trouble of disconnecting the bat- 
tery from the coil when it is desired to turn 
current off may be avoided. 

The tube should be placed in the adjustable 
stand d,nd the two wires from the secondary ter- 
minals of the coil connected to the proper elec- 
trodes. The wires connected to the tube should 
be of very small cross section, say, from No. 26 
to No. 29, and need not be twisted or securely 
fastened to the terminals of the tube. It will be 
sufficient to hook them on loosely. If large wires 
are used to connect to the tube terminals there is 

148 The A B C of the X Rays. 

danger of cracking the glass and thus destroying 
the vacuum. 

If the experimenter has provided a spark gap 
in the wires leading from the secondary tierminals 
of the coil, as will be shortly described, the tube 
is, of course, connected with the wires leading 
from this spark gap. If it is desired to make a 
fluoroscopic examination the tube should be 
so placed in the adjustable stand that the cathode 
electrode will face the observer if the tube is 
one of the non-focussing type. If the tube is of 
the focussing type, it should be so placed in the 
stand that the anode faces the observer. (See 
Fig. 32). 

The current may now be turned on in the coil 
and the electrical discharges will take place 
through the tube. If the tube has been properly 
connected and is producing X rays, their effect 
will be perceptible by the illumination of the 
screen in the fluoroscope when it is placed im- 
mediately in front of the Crookes tube. Objects 
may be examined by placing them closely in 
contact with the outside of the screen when it is 
held in this position. The open end of the fluor- 
oscope should be pressed closely against the face 
to exclude any outside light. 

While fluoroscopic examinations can be made 

The A B C of the X Rays. 149 

in a liglit room, the experimenter will always 
find it mucli more advantageous to have a room 
partially or wholly dark for the purpose. When 
the fluoroscope is used in a light room and the 
operator removes it from the eyes it takes per- 
haps ten or more minutes on renewing the ex- 
aminations, to get the eyes back to the proper 
condition for making accurate observation of the 
fluorescent screen, as the eye is very slow to 
recover full sensitiveness. 

The experimenter may not get much, if any, 
fluorescence upon first connecting up the tube to 
the coil. There may be two reasons for this : (i) 
that two connecting wires may be in contact 
with each other, and (2) that the anode and 
cathode terminals of the coil may not be con- 
nected to the proper electrodes of the tube. The 
remedy for either of these troubles is obvious. 

If the experimenter now desires to take a 
radiograph, the current should be shut off from 
the coil until the sensitized plate has been placed 
in position. The tube should be so adjusted that 
the X rays will fall at the place where it is in- 
tended to locate the plate. 

The plate may be placed either in an ordinary 
plate-holder, which should be closed in the 
regular manner with a slide (not of metal), or 

150 The A B C of the X Rays. 

the plate may be wrapped up in a piece of pho- 
tograplier's black paper, with sufi&cient folds to 
exclude light both from the sides and the ends. 
In so arranging the plate, care should always be 
taken that it can be so placed that the film side 
will be uppermost. The reason for this is that 
the nearer the object to be radiographed is to the 
film, the clearer the definition will be. 

The plate as thus arranged may be placed on 
the table with the film uppermost to that point 
of the tube which shows the strongest effects of 
the X rays. 

The object to be radiographed is so arranged 
that it will be at a distance of from, say 4 to 12 
inches away from the tube and as close to the 
plate as possible. The current may be turned 
on and the exposure made. The current should 
never be turned on after this sensitized plate is 
brought near the Crookes tube until the operator 
has the object placed in position and is ready to 
make the exposure. After the exposure has 
been made the plate is developed in the usual 
manner as if an ordinary photograph had been 
taken upon it. 

No specific time can be laid down as to the 
time of exposure that shall be made. With a 
small coil giving only one inch spark and a 

The A B C of the X Rays. 151 

focussing tube, a radiograph of a liand will prob- 
ably be made in twenty minutes and of coins in a 
purse in nine or ten minutes. As tbe exciting 
apparatus increases in size and power the time 
of exposure grows less. As, for instance, with a 
coil giving a six inch spark and focussing tube, 
a good radiograph of the hand may be made in 
two to two and a half minutes, while with a coil 
giving a 12 inch spark and a double focussing 
tube a similar picture can be obtained in about a 
minute or perhaps less. 

The time of exposure varies so much with the 
class, quality and power of apparatus used, that 
it is practically impossible to say definitely just 
what exposure shall be given in any specific 
case. This can only be determined in each case 
by the results of previous experiments. 

Sometimes the experimenter may desire to 
stand his plate-holder or protected plate on edge 
to take a radiograph of some object. It should 
be borne in mind that objects may be tied fast to 
the plate-holder by means of thread or twine or 
even fastened thereto with rubber bands, but not 
with rubber tape. These fastenings will not 
show in the radiograph, as the X rays pass en- 
tirely through them and leave no shadow of them 
upon the plate. 

152 The A B C of the X Rays. 

Before dealing witli the manipulation of tlie 
larger sizes of exciting apparatus we sliall sug- 
gest to the reader more in detail the idea of the 
variable spark gap in the circuit leading from 
the secondary terminals of the induction coil. 
This spark gap is usually arranged for in high 
frequency apparatus and is also frequently used 
in operating static machines. 

In Fig. 33 will be found an illustration of one 

Fig. 33. 

AA, Wires leading to discharge terminals of coil ; BB, Wires leading 
to tube. 

practical method of making the variable spark 
gap. It will be seen that there are four stand- 
ards arranged in two pairs upon a board. Bach 
of these pairs has the ends of the standards bent 
and pointing toward each other, but with a gap 
between them. 

The A B C of the X Rays. 153 

The standards themselves may be four pieces 
of wire with the bare ends bent over as shown in 
the illustration. The standards should always 
be further apart than the greatest length of the 
spark capable of being given by the coil. The 
board in which the standards are mounted should 
be kept dry, and may be preferably mounted 
upon small blocks of glass, hard rubber, or other 
good insulator. It will be seen that on a stand- 
ard at each end is connected a wire leading from 
one of the secondary terminals, and on the other 
two standards are thin wires leading to the Crookes 

The above is a suggestion for a rough-and- 
ready home-made spark gap. There are regular 
discharge stands made for the purpose, much 
more convenient to handle, now made and ob- 
tainable from dealers in X ray apparatus. These 
usually consist each of a small wooden stand in 
which is inserted a stout wire surmounted by a 
ball. The wires leading to the Crookes tube 
are fastened to these wires and the length of the 
spark gap can be regulated by moving the 
wooden stand. 

In practice, it may be found unnecessary to have 
the two points in the spark gap separated. If, 
however, the vacuum tube does not appear to be 

1 54 The A B C of the X Rays. 

giving its best results, the points of separation in 
the spark gap may be increased, until after 
several trials the best results from the tube are 
obtained. It is not usually necessary to have a 
separation of more than an inch to an inch and 
a half as the extreme limit. With continual use 
a Crookes tube will rise higher in vacuum, and 
on slight increases of this kind the variation of 
the spark gap will be found very useful and con- 
ducive to better results. Vacuum tubes vary 
quite a great deal from each other, even when 
made by the same manufacturer, and each one 
will be found to have its individual peculiarities. 

It has been noticed also, that some tubes will 
vary slightly from day to day. It has been 
found, however, that this spark gap is very use- 
ful in meeting such variations in most cases. 
Of course, if a tube rises so high in vacuum that 
no discharge can be sent through it, there is 
nothing to do but to have it re-exhausted, unless 
it is a tube of the adjustable vacuum kind, in 
which case the vacuum can be lowered in a few 

Returning to the methods of manipulation, 
and taking up exciting apparatus of considerable 
power and capacity we find that the practical 
manner of taking a radiograph or making fluoro- 

The A B C of the X Rays. 155 

scopic examinations is substantially the same as 
detailed above for small coils. The chief diffi- 
culties will be found in the time of exposure 
necessary, the distance of the objects from the 
tube and in the method of making and breaking 
the circuit. 

As we stated above, it is difficult to make 
specific recommendations as to the length of time 
of exposure in taking a radiograph of any object. 
Objects of small size, will, of course, require a 
very much shorter time of exposure than large 
or dense objects, although, in any event, the 
time of exposure increases with the distance of 
the tube from the object. Of the various parts 
of the human body, the hand and -the adjacent 
ends of the arm-bones require a comparatively 
short time of exposure. A foot, ankle or other 
parts of the limbs require usually about three 
times as long an exposure for a radiograph of 
equal quality. To obtain pictures of portions of 
the trunk usually takes fully ten times as much 
exposure as for the hand and, even then, the ap- 
paratus must be in first-class working condition 
and an abundance of X rays generated. 

The distance of the object radiographed by 
means of apparatus giving sparks of six inches 
and upwards should be six inches or more from 

J 56 The A B C of the X Rays. 

tlie tube. This, of course, refers to such parts 
as the hand, arm or foot, where the distance of 
the tube may be as great as twelve inches or 
more. In taking radiographs of the limbs it 
would be advisable, generally speaking, to have 
the tube only six or eight inches away to obtain 
a picture with the minimum length of exposure. 
A still shorter exposure can be made of such a 
part if only a very limited portion is desired to 
be shown on the plate by placing the tube two 
or three inches nearer. It should be remembered, 
however, that the greater the distance from the 
tube, the clearer the definition, although the ex- 
posure must be longer. 

In taking a radiograph of a portion of the 
trunk, it is not always practicable to have the 
tube a long distance away from the body, as an 
exposure of several hours might, in such a case, 
be necessary. 

In taking radiographs through the human 
trunk or through the thicker parts of the limbs, 
it is usual to shorten the time of exposure by 
placing a fluorescence screen between the object 
and the sensitized plate. The effect of this is to 
place next to the plate a screen which is rendered 
fluorescent by the X rays except in the parts 
where the rays are intercepted by the opaque 

Fig. 34- 


of Hand with Needle in Palm, made by the author with Thomson Double- 
focus Tube. 
Time of exposure 2 minutes 20 seconds. 
Distance from Tube 10 inches. Tube excited with Thomson Inductorium 
on 5 inch spark. 

The A B C of the X Rays. 157 

body, and the consequence is that the plate is 
affected by the light parts of the screen while 
the opaque parts will appear dark when the plate 
is developed and printed. 

Soraetinies it may be desirable to use a flexible 
sensitized photographic film tied over the part to 
be radiographed, as in the case of the hand con- 
taining a needle, shown in the illustration, Fig. 
34. This hand was radiographed by the writer. 
The fingers were partially contracted and could 
not be opened, and the flexible film was tied to 
the inner portion of the hand by means of thread 
and a radiograph made by allowing the X rays 
to fall on the back of the hand. 

While with small coils the ordinary form of 
vibrating contact breaker will answer most pur- 
poses for amateur experiments, it is advisable to 
use rotary contact breakers on those giving 
sparks of five inches and upwards. The rotary 
contact breaker is much more certain, rapid and 
satisfactory in its action than vibrating contact 
breakers, and it may be considered to be the only 
form desirable for use in practical X ray work. 
Its use is not necessarily confined to coils of a 
large size. It can be used in connection with 
coils giving one inch spark and upwards by dis- 
connecting the vibrator parts and connecting in 

158 The A B C of the X Rays. 

place of them the rotary circuit breaker. This, 
however, will only commend itself to those who 
desire to go beyond the limit of the range of 
amateur experiments. For such it may be well 
to state that rotary contact breakers can be had 
in the market and are provided with motors that 
are capable of use either with batteries or upon 
electric lighting circuits.* 

It will probably be of interest to the reader to 
anticipate some of the troubles that may possibly 
arise in connection with his work or apparatus. 
It cannot be expected that, with constant use, 
apparatus of this kind will always work smoothly, 
but such troubles are not without their compen- 
sation, as it is by them that we most frequently 
gather our most valuable information. 

We will suppose, for instance, that the reader 
is making experiments with a small coil having 
the regular form of vibrating contact breaker. 
Some day on connecting his tube he does not 
get the usual results. It is best then to begin 
with the coil, disconnecting the tube for that 
purpose and adjusting the discharge points of 
the secondary terminals to see if the coil is 

* Since the above was written there has been devised and perfected a 
special form of vibrating contact breaker which works as perfectly and 
practically as the rotary contact breaker. 

The A B C of the X Rays. 159 

producing its usual length of spark and of the 
same quality. 

If the spark is irregular and smaller and 
thinner than it should be, it is possible that 
the trouble may be with the contact points 
upon which the vibrator works. These should 
be bright and clean and make good contact. If 
these are found in good condition, possibly the 
battery may need renewal of solution. If when 
this is tried the coil does not still work well, all 
the connections throughout should be examined 
to see if they are clean and firm, and if it then 
does not work, there is a possibility that the coil 
may have been injured in its insulation by be- 
ing pierced with a spark. 

If the coil is found to be all right the connec- 
tions with the tube may be renewed and another 
trial made, and if the tube does not work well 
and if all.the connections are clean, clear and prop- 
erly made, and the tube still does not work well, 
the variable spark gap should be tried with all 
the variations that can be thought of, including 
the sparking of one polarity at a time. If an 
adjustable condenser is being used the various 
combinations of adjustment may be tried to ad- 
vantage. Should the trouble appear to be with 

i6o The A B C of the X Rays. 

the tube there may be one of two reasons, 
namely : — 

1. .The tube may be cracked or punctured. In 
this case it will be filled with a dark blue or purple 
cloud, or the spark will jump from one electrode to 
another inside of the bulb. Should the tube 
present either of these appearances, it is, beyond 
question, punctured or cracked, and nothing can 
be done with it for production of X rays until it 
has been repaired. If there is a thin, light blue, 
cloudy appearance around the electrodes, that is 
the indication of a low vacuum, especially if a 
slight fluorescence appears on some portion of 
the glass. This may be remedied by running 
the electrical discharges through the tube for 
some time (with spark gap closed) say, five or 
ten minutes, and letting the tube cool off for an 
equal length of time. Several trials of this kind 
should bring the tube up to proper fluorescence 
and put it in condition to produce the X rays. 

2. The tube may have become too high in 
vacuum. In this case the electrical discharges 
will not pass through the tube ; it will remain 
dark and little or no fluorescence of the glass 
will appear. Sometimes, the discharges from 
the coil will cause sparks to pass from terminal 
to terminal outside of the tube when the vacuum 

The A B C of the X Rays. i6i 

is too higli. There are two remedies for this, 
unless the tube in question happens to be one 
with an adjustable vacuum, one remedy being to 
gently heat the surface of the tube with a spirit 
lamp or Bunsen burner, and the other is to send 
it to the manufacturer for re-exhaustion. 

If the foregoing does not seem to cover the 
trouble with the tube, it will be best to lay it 
aside for several days or perhaps longer. The 
writer has known Crookes tubes to work badly 
under apparently proper conditions, but after 
being laid aside for a week or ten days to work 
as well as they ever did. 

The above remarks as to troubles that may 
possibly arise with tubes on small apparatus are 
applicable to tubes used on large apparatus. The 
same close search for broken circuits or bad con- 
tacts should also be made. If the operator is 
using a large induction coil with rotary contact 
breaker the trouble may be found in the brushes 
by reason either of their not bearing smoothly 
upon the metallic surfaces, or in the brushes 
having worn down irregularly, or the metallic 
contact surfaces of the contact breaker having 
become very dirty or very much cut up. There 
is also another possible source of trouble which 
may arise from small particles of metal having 

1 62 The A B C of the X Rays. 

been carried over tlie insulating surface of the 
contact breaker, thus destroying the complete- 
ness and efficiency of the break. 

It is not intended to discourage the reader by 
these remarks, but to look forward to what may 
possibly happen, but what does not frequently 
occur in actual practice. Generally speaking, it 
is desirable to see that all parts of the apparatus 
are kept clean and free from dust and dirt and 
that all contacts are properly made and kept in 
good condition so that the maximum results may 
be usually obtained. 

Ordinarily, when X ray apparatus is purchased 
from a reputable concern, instructions for setting 
it up and operating it should be furnished, espe- 
cially if the apparatus is of an unusual capacity. 
Inasmuch, however, as X ray experiments may 
be made with a class of exciting apparatus that 
has been standard for many years and as a great 
many persons already have apparatus of this 
kind it may be of some assistance to show how 
such apparatus can be connected with the elec- 
tric lighting circuit. 

The diagrams Figs. 35 and 36 will show how 
this may be done. It will be noted in Fig. 35 
that one conductor from the mains of the elec- 
tric light circuit is connected to one of the primary 

The A B C of the X Rays. 


coil terminals. In a branch from the other main 
is placed a rheostat or a bank of lamps for resist- 
ance, the contact breaker being placed between 
the resistance and the other primary terminal. 




Fig. 3S. 

AA, Main line ; BB, Branch line ; C, Bank of lamps ; D, Rotary 
contact breaker ; E, Condenser ; FF, Terminals of secondary coil. 

It will be seen that the condenser is in shunt 
around the circuit breaker. 

In the diagram, Fig. 36, the same general 
arrangement is shown except that a set of storage 


The A B C of the X Rays. 

batteries is arranged across the circuit. This 
need not be the regular form of storage battery, 
but each cell may consist simply of two lead 
plates immersed in a solution of one part of sul- 
phuric acid to ten parts of water. Of course the 
lead plates in each cell should be kept separate 
from each other and all of the cells connected in 
series, as shown in the diagram. 

Fig. 36. 

A A, Main lines; BB, GG, Branch lines; C. Bank of lamps; D, 
Storage cells ; E, Rotary contact treaker; F, Condenser; HH, Termi- 
nals of secondary coil. 

In operating an induction coil upon the electric 
light circuit according to the methods shown by 
the diagrams, the amount of electrical energy 
supplied to the coil may be regulated by the 
rheostat or by the number of lamps allowed to 
burn in the bank of lamps used for resistance. 

The A B C of the X Rays. 165 

Kvery lamp connected will of course allow more 
current to flow to the coil. 

In using a new vacuum tube, it is advisable to 
keep down the amount of current flowing through 
the primary coil so as to bring the tube gradually 
up to its full capacity. This is especially the 
case if the type of tube used is not one of the 
focussing kind. Many a tube is spoiled by dis- 
charging through it when it is first used the full 
capacity of the coil when excited with the maxi- 
mum current upon which it is used. 

Frequently a new Crookes tube will show a 
very light blue cloudiness around the electrodes, 
and this should be worked out gradually by first 
operating the tube only with a small current. 
While it is best to do this either with focussing 
or non-focussing Crookes tubes, it is not so 
necessary with the focus tube as it is with the 

The method of taking radiographs or making 
visual examinations with the fluoroscope in con- 
nection with high frequency apparatus or with 
Holtz or Wimshurst static machines is practi- 
cally the same as when these operations are per- 
formed in connection with induction coils. That 
is to say, the tube is placed in the stand, and 
when the X rays are generated examinations 

1 66 

The A B C of the X Rays. 

^av be made with the fluoroscope, or radiographs 

av be taken in the same general manner as 

described above. The remarks already made m 

regard to the time of exposure and the distance 

Fig. n. 
Crookes tube in adjustable stand. 

of the tube from the object are, in the main, ap- 
plicable also when these other types of exciting 
apparatus are used. 

Supplementing what has already been said in 

The A B C of the X Rays. 167 

regard to distance and time of exposure, we 
desire it to be understood that tlie examples 
given are only intended to be approximate. 
These points can only be definitely settled by 
the operator after several trials. It is impossible 
to specify the exact time and distance requisite 
in any particular case unless the density of the 
object, and the degree of perfection of apparatus 
and the strength and abundance of the X.rays 
are known. It will be apparent, therefore, that 
the above remarks on these points are intended 
merely as a sort of starting-point and some de- 
gree of guidance for the operator. A very short 
experience and practical use of X ray apparatus 
will serve as guide in determining the condi- 
tions necessary to obtain the best effects. 

It is scarcely possible to give any specific 
directions as to the care and handling of static 
machines and high frequency apparatus, as they 
differ in their arrangement and construction as 
made by the various manufacturers. All repu- 
table makers furnish with their machines the 
necessary instructions for their proper care and 
operation, and, as these are founded iipon expe- 
rience with their own apparatus, the purchaser 
will usually be able to obtain good results by the 
exercise of intelligent care and attention. 

1 68 The A B C of the X Rays. 

We believe that the amateur experimenter will 
have little trouble in obtaining satisfactory radi- 
ographs and fluoroscopic work with any good 
type of exciting apparatus tipon following the 
hints contained in this chapter. 

The A B C of the X Rays. 169 



The readers of this book will probably be of 
two classes, namely, tbose wbo are merely seek- 
ing for general information on the subject of X 
rays, and those who are desirous of either exper- 
imenting, investigating or making practical use 
of the results. The contents of this chapter are 
intended to be of some practical use to the latter 
of these two classes of persons. 

Beginning with the amateur experimenter who 
may already possess some one or more of the 
small types of apparatus that can be used in these 
investigations, let us see how far he may be able 
to go. There are many thousands of amateur 
experimenters who already possess induction 
coils and static machines that are capable of 
furnishing sparks of at least one inch in length. 
Prom the items that have appeared in the news- 
papers from time to time it might be thought 
that such modest apparatus would be of insuflE- 

1 70 The A B C of the X Rays. 

cient power to produce any satisfactory results 
in experiments with the X rays. It is our priv- 
ilege, however, to be able to offer more encour- 
agement to the possessors of apparatus of this 

While it is, as a matter of fact, possible to ob- 
tain some results with exciting apparatus giving 
less than one inch spark, the use of such small 
apparatus cannot be confidently recommended to 
the experimenter. The possessor of an induc- 
tion coil or static machine, either Holtz or Wims- 
hurst, giving sparks of one inch and upwards, 
can, however, make many interesting investiga- 
tions and experiments therewith in connection 
with a small single focus tube. 

Should the reader desire to make these ex- 
periments with an induction coil giving a one- 
inch spark, it is suggested that either one or two 
cells of storage battery or of a good primary bat- 
tery, such as the Grenet, Bunsen, Fuller or 
Grove, or four cells of Bdison-Lalande battery, 
be used to actuate the induction coil. The Le- 
clanche cell is scarcely, suitable for use with an 
induction coil for X ray work. Gravity cells 
may be employed, but in this case it would be 
necessary to connect from six to twelve of them 

The A B C of the X Rays. fjt 

in two multiple series of three each for an in- 
duction coil of this size. 

A single focus tube is by all means the best 
type of Crookes tube to be used in connection 
with a small induction coil or static machine, as 
the stream of cathodic rays can be concentrated 
and brought to a focus in the tiibe, and, proceed- 
ing in straight lines from the point of focus, will 
ensure sharply defined outlines of the opaque 
objects examined under the fluoroscope or laid 
upon a sensitized photographic plate. A spe- 
cially designed single focus tube for small coils 
is made by some of the manufacturers and can 
readily be obtained at a moderate price. 

With a small induction coil or static machine 
of the kind above indicated and one of these 
single focus tubes, together with a fluoroscope 
or fluorescence screen and some ordinary photo- 
graphic plates, the experimenter is eqtiipped for 
making observations and radiographs by means 
of the X rays. With such a set of apparatus, 
the whole cost of which is quite modest, very' 
fair radiographs of small objects can be made and 
fairly good results can he obtained with the 

It will be found that with apparatus of this 
kind, radiographs of objects can be made through 

172 The A B C of the X Rays. 

several thicknesses of wood, or heavy millboard. 
Of course, the time of exposure would necessarily 
be a little longer than if a larger coil or static 
machine were used. 

It will be quite apparent, therefore, that if an 
ind'uction coil giving an inch spark can be suc- 
cessfully used, coils giving a greater length of 
spark may naturally be expected to produce cor- 
respondingly better results. There are very 
many persons possessing induction coils of what 
may be stated as of medium capacity, such as 
for instance those giving in the neighborhood of 
two or three inch sparks, who are still uncertain 
whether or not they can make experiments with 
X rays by means of these coils. 

It is quite important in such work to use a 
battery which has a good output in amperes. 
The storage battery is an ideal one in this respect, 
but these are not always convenient on account 
of the necessity of recharging them from some 
source of direct current. Next in desirability is 
the primary battery known as the Edison-Lalandc 
cell, which has a very large ampere capacity and 
almost no polarization when it is not in use. 
The electro-motive force of this cell is compara- 
tively low, however, being only about seven- 
tenths of a volt per cell, while the storage bat- 

The A B C of the X Rays. 173 

tery, the Grenet, Bunsen, Grove and Fuller cells 
give in tlie neighborhood of two volts per cell. 
Although the use of the Edison-Lalande cell 
would necessitate the employment of a larger 
number, it is certainly the choice as between the 
various kinds of primary batteries for the class 
of work under consideration. 

It will generally be found suflEcient for induc- 
tion coils giving sparks of from one to two inches 
to connect two cells of storage battery or six 
cells of the Edison-Lalande battery to obtain the 
proper working capacity of the coil. If any of 
the other types of primary battery are used, it 
depends on the size of the cell as to how many 
would be required to get the same effect. A sin- 
gle cell Grenet, Bunsen, Grove or Fuller Battery 
of the size contained in a one-gallon jar would 
give practically as good results as one medium 
size cell of storage battery, but if smaller cups 
of primary battery were used it would be advis- 
able for the experimenter to have more than one 
set of cells on hand so that two or more can be 
connected in multiple to obtain the aggregate 
amperes of the cells so connected. For an induc- 
tion coil giving three or four inch sparks, it 
would be advisable to double the battery capac- 
ity which we have just now stated for the small 

1 74 The A B C of the X Rays. 

coil, or, at any rate, to have them connected ready 
to be thrown in circuit if necessary. 

The fortunate possessor of a static machine is 
not troubled with the questions of battery, as the 
source of the electricity is then within the ma- 
chine itself and brought up to its maximum by 
the revolution of the plates. The proper excita- 
tion of the small focus tube with a moderate size 
static machine is a very simple operation as there 
are no batteries to be cared for. 

All static machines are alike in this respect, 
namely, that they contain within themselves the 
source of their own current, and whether the 
machine be large or small, it only depends upon 
the operator to provide for some means of revolv- 
ing the plates. In the larger sizes of static 
machines, a small motor of some kind is an ex- 
ceedingly desirable adjunct, as the plates are re- 
volved thereby with greater regularity and less 
labor than they can possibly be operated by 

So far as the results are concerned, as between 
static machines and apparatus of the induction 
coil type, it is doubtful whether there is any 
choice for one who is providing himself with a 
set of apparatus for X ray work. Equally good 
results can be obtained, under proper conditions, 

The A B C of the X Rays. 175 

with eithei" class of apparatus of about tlie same 
degree of power in spark discharge capacity. 
The choice of the type of apparatus is very 
largely a question of individual preference, hav- 
ing due regard to the conditions under which 
the buyer expects to operate it. 

Taking up for consideration the necessities of 
those who desire to make more extensive and 
elaborate investigations of the X ray phenomena 
and to make practical use thereof, we find that 
induction coils of larger sparking capacity will 
be required if that is the class of apparatus de- 
cided upon. A good size of coil for such pur- 
poses is one that will give about five or six inch 
sparks of good quality, in fact as we have before 
stated, a fat, heavy spark is far more desirable 
■for this work than a long, thin, lean spark, al- 
though some results can be obtained from a spark 
of the latter quality. For the use of the physi- 
cian or sitrgeon, an induction coil or apparatus 
giving about this length of spark will be found 
to be most generally useful for nearly all pur- 
poses, especially if a Crookes tube of good design 
and high efficiency be used in connection there- 

We are again confronted here, however, with 
the question of individual preference, govemec^. 

176 The A B C of the X Rays. 

by the existing conditions surrounding the con- 
templating purchaser of apparatus. As stated 
above, an exciting apparatus having a spark of 
about five or six inches is suflBcient for almost all 
kinds of X ray work. There are several types 
of exciting apparatus obtainable giving this size 
of spark. One is the Induction Coil, another is 
the Static Machine, and a third is the High Fre- 
quency apparatus for use on an alternating cur- 

To operate the apparatus it is necessary, in 
the case of induction coils, to have either bat- 
teries or current from an electric light circuit 
supplying direct current. The static machine 
can be operated either by hand or a small motor, 
deriving its power either from electricity, gas or 
water, while the high frequency, or transformer, 
apparatus can be operated only from an alternat- 
ing current circuit. 

Assuming that the spark length of feach of the 
above named three classes of apparatus is about 
equal, it should be noted that the high frequency 
apparatus is the most powerful of either of these 
three types. If, therefore, a physician or other 
person contemplating the purchase of apparatus 
is so situated as to be convenient to an alternat- 
ing current circuit, the writer recommends the 

The A B C of the X Rays. 177 

adoption of exciting apparatus of the trans- 
former type. 

It is Highly efficient for the purpose, requires 
but few accessories and may be connected direct 
to the circuit, thus avoiding the use of batteries. 

Should there be no alternating current circuit 
available, the choice will necessarily be between 
an induction coil and a static machine. We 
have already pointed out that the former can be 
operated either directly from an electric light 
circuit, or with a few cells of storage battery, or 
a combination of both, while a static machine 
may be operated either by hand or by means of a 
small motor. When either type of apparatus is 
once set up and ready for operation there is but 
little difference in the amount of manipulation 
required to operate either one of them. Of the 
two the static machine requires, perhaps, the least 
number of accessories for its operation and pro- 
duces very sharply defined radiographs. The 
induction coil, on the other hand, is generally 
regarded as being more flexible in the variations 
which can be obtained from it, and it is, if any- 
thing, less delicate in construction and arrange- 
ment and perhaps is an apparatus that will stand 
more years of constant usage. 

As we have stated above, it is, however, largely 

1/8 The A B C of the X Rays. 

a matter of individual preference, and if the 
reader should choose either type of apparatus of 
the capacity in question and purchase it from a 
reliable manufacturer, he may rely on being 
able to produce satisfactory results if he operates 
it with judgment and discretion. 

If it is desired to do the very highest kinds of 
X ray work, it may be advisable at the .outset 
to invest in a larger induction coil or static 
machine. These are made commercially to give 
from 8 to 12 inch sparks, and may be operated 
in the same way as the smaller ones, but requir- 
ing increased power, of course. Apparatus of 
still larger sparking capacity can also be ob- 
tained, but thus far it has not been necessary to 
employ any giving sparks of greater than 12 
inches in length for the highest X fay effects. 

The A B C of the X Rays. 1 79 


The art of photography is so well understood 
in these days that it seems almost superfluous to 
offer any suggestions on this line. As experi- 
ments with X rays will, however, in many cases, 
be made by those who have had no experience 
whatever in the art of photography, we shall 
venture to offer a few suggestions to the reader 
who may never have handled or developed a 
photographic plate. 

To take a radiograph by means of the X rays, 
a sensitized photographic plate is, of course, a 
necessity. These are obtainable in almost any 
city or town. The first necessity is to have a 
dark room and the next a ruby light. Lanterns 
with ruby glass panes can be bought at any 
photographic supply store. An undeveloped 
photographic plate should never be exposed to 
any but a ruby light. 

Let us suppose that the experimenter has pro- 

i8o The A B C of the X Rays. 

vided himself with a package of plates and a ruby 
light. He should go into a perfectly dark room, 
light the lantern, and close its opening. The 
box of plates may then be opened. A plate is 
taken out and should be either placed in a 
regular plate-holder, or wrapped up in photo- 
grapher's black paper, or placed in a cardboard 

A photographic plate has upon one side of it 
a hard, white, sensitized gelatine film. The 
film side can always be ascertained by holding 
the plate sideways towards the red light. The 
film side is dull while the other side of the plate, 
being plain glass, has a bright and shining ap- 
pearance. If still in doubt, the film side can be 
detected by moistening a finger and touching one 
corner of the plate. If the film is on that side, 
it will feel sticky. 

In placing a plate in the holder or any other 
covering, great care should be taken that the 
film side is uppermost, as it is desirable to have 
the film as near as possible to the object to be radio- 
graphed. The box of ^plates should be closed 
again and the operator is ready to take the radio- 

If the plate is placed in a cardboard box, the 
box should be filled up with several papers or a 

The A B C of the X Rays. i8i 

few corks so as to insure the film being near the 
upper surface of the box. 

It is not necessary to open the plate-holder or 
other covering of the plate to make the exposure, 
as it can be made through the cover of the plate- 
holder or through the envelope covering the plate. 
In purchasing- a plate-holder one should be 
selected with a sliding cover that is not made of 

The exposure having been made, the next 
thing in order is to develop the plate. To do 
this and fix the image, two things are necessary : 
one a developing solution and the other a solu- 
tion of hyposulphite of soda. The latter solution 
is for the purpose of fixing the image upon the 
plate after development. 

Two trays to hold the solutions are necessary, 
one for the developer and one for the hyposul- 
phite of soda. Neither of these trays should ever 
be used for any other solution than the kind first 
put in it. Single solution developers can be pur- 
chased of all dealers in photographic materials, 
who also sell the hyposulphite of soda. The 
latter article is in a crystal, and must be dis- 
solved in water, according to the formula which 
will be found enclosed in any box of plates. 

It will be seen, therefore, that only two bot- 

1 82 The A B C of the X Rays. 

ties will be necessary, one a developing solution 
and the other a solution of hyposulphite of soda. 
We will now suppose that the operator is ready 
to develop the picture. 

A small portion of the developing solution is 
mixed with cold water, according to the direc- 
tions usually found on the bottle. This diluted 
solutibn is poured into a tray and the plate 
placed in it, film side up. This is all done in the 
ruby light, of course. The tray is gently rocked 
from side to side for a few minutes and the image 
will slowly appear. The plate should be kept in 
the solution until it (the plate) has become quite 
dark, and until the image has apparently almost 
disappeared. It should then be taken out and 
washed in clean water and then put into the 
other tray containing the solution of hyposul- 
phite of soda. 

At this time the plate, at the back, is of "a light 
color, almost the same as it originally had, but 
after being in the solution of hyposulphite of soda 
for five or ten minutes this will disappear and the 
plate will look black. The picture is then de- 
veloped and fixed, and may be taken out into the 
light, but before being set up to dry should be 
washed for about half an hour in running water, 
or if that is not available it should be washed in 

The A B C of the X Rays. 183 

clean cold water for about the same length o£ 
time, the water being changed very frequently. 
Cold water should be used throughout. 

The operation is less formi,dable than it might- 
appear from reading the above. After a few 
pictures have been taken it will be found that a 
radiograph can be made, developed and fixed in 
fifteen or twenty minutes. If the reader has not 
had any previous photographic experience, he is 
recommended not to try an important picture as 
his first radiograph, but to experiment a little 
before proceeding to that class of work. It is 
exceedingly simple primarily, but, of course, 
pictures of a high degree of excellence are only 
obtained by experience. 

The best kind of photographic plates for X 
ray work are those which have a heavy coating 
of film. These are specially made for this work 
by some of the well known manufacturers. Any 
good developer may be used, but for general 
work, and especially for amateurs, "Metol " devel- 
oper is strongly recommended and has been used 
by the writer with considerable success. The 
developer put up in dry form by J. Carbutt of 
Philadelphia, and know as the "J. C. Tabloids," 
has also been much recommended by many 

i84 The A B C of the X Rays. 

persons who have made large numbers of radio- 

For the convenience of the reader it is also 
suggested that t^e same concern furnishes 
rjpecially prepared photographic plates for X ray- 
work, each plate wrapped separately and sealed 
in black paper. These are known as Carbutts' 
X ray plates and may be obtained from regular 




Accumulator (Condenser) 66 

Adjustable Vacuum Crookes tubes 127 

Air blast (Higli Frequency apparatus) 85 

Alternating current apparatus 74 

Action of 81 

Alternations of current 76 

Apparatus for producing X Rays 15-29 

Armature — vibrating 45 

Attraction and repulsion 91 


Batteries i44, 170, i73 

Bound electricity 93 


Coils. See induction coils. 

Condenser 53, 66 

" action of 72 

Contact breaker 45, 52, 60, 157 

" " Essentials of 61 

" " Rotary 62 

Connections pf double-focus tube 122 

Crookes tube 17, 106 

" " On electric light circuit 29 

" " Qualities of exciting current 29 

" " Edison pattern 116 

1 86 Index. 


Crookes tube, Thomson pattern 119 

" " Double-focus 120 

" " Single-focus 124 

" " Adjustable vacuum 127 

" " Loss of vacuum 160 

Core for induction coil 48 

Current, direction of 36 

' ' source of 136 

Cycles 77 


Definition 88 

Developing a photographic plate 181 

Developers, photographic 183 

Discharger 71 

Discovery, time of Prof. Roentgen's 10 


Edison Crookes tubes 116 

Electric light circuits, connections with 163 

Electrical energy for Crookes tube 31 

Electromagnetic current induction.. 39 

Electrophorus 94 

Exciting apparatus 29 

" " Means for operating 137 


Fluorescence 25 

Fluoroscope 26, 129 

Fluorescent screen 27, 131 

Fluorescence of Crookes tubes 109 

Fluorescing crystals 130 

Fluoroscope, use of 132 

Index. 187 

J^ocusing Crookes tubes 119 

Thomson double-focus 120 

Single-focus 124 

Frequency 77 

Free electricity 93 


Gas cells 164 

Geissler tubes 107 

High Frequency apparatus 74 

" " " Means of operating 139 

Holtz Static Machine 98 


Induction coil, character of sparks 15 

Induction, theory and philosophy 34 

Inductive effects. 40 

Induction, self 42 

Induction coils 47 

" " Corefor 48 

" " Primary coil 49 

" " Secondary coil 50 

" " Contact breaker 52 

" " Condenser 53 

" " Pole changer 54 

" " Late improvements in 56 

" " Insulation, oil 58 

" " Current for opera;ting 141 

L,eyden Jar 70 

Light rays, refraction of . 14 

.'tiass of forae.>< ••- 36 

1 38 Index. 



Magnetic field 35, 37 

Manipulation 146 

Mechanical contact breaker 61 


Oil insulation for coils 58 


Penetration of the X rays 23 

Photograph 20 

Photographic plates and developers 179 

Pole changer for induction coil 54 

Practical suggestions 169 

Pressure of induced currents 43 

Primary coil 49 

Primary wire 41 


Radiograph 20 

' ' making a 149 

" time of exposure 151, 155 

Regulator for High Frequency apparatus 86 

Repulsion of similarly electrified bodies. gi 

Rotary contact breaker 61 

Ruhmkorff coil. See induction coils 

" " Current for operating 141 

" " Character of sparks 15 


Secondary wire 41 

Self induction 42 

Secondary coil 49 

Source of current 136 

Spark length 44 

Index. 189 


Spark gap (High Frequency apparatus) 84 

Spark gap 152 

Static Machines 90 

Holtz 96 

" " " Action of 99 

" " Wimshurst 104 

" " Means of operating 138 ■ 

Storage batteries for operating coils 143 

Suggestions, practical 169 


Transformers— High Frequency 74 

" " " operation 139 

Troubles with apparatus 159 

Tubes low vacuum 16 

" Crookes 17,106 

" Geissler 107 

Edison iii6 

" Thomson 119 

" " double-focus 120 

" Single-focus 124/ 


Vacuum, adjustment of in Crookes tubes 127 

" Degree of in " " 108 

" Rise 6f in " " 125 

Vibrating contact breaker 45 


X Rays, ascertaining presence of 18 

" Effect of- on sensitized plate 22 

" Generation of 12, 17 

* " Penetration of 23 


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